BLUE TUTORIALS

Chapter introduction
Introduction
more and more,it is networks that connect us.people communicate online from everywhere.
convercations in cofeehouses spill into chat rooms. on line debates continue at school
efficient,relieble technology enables neetworks to be available whenever and wherever we need them
in this chapter you will learn how communication occursover a network and about the many different components that need to operate together to make it work
after completion of this chapter, yuo should be able to:
-explain the concept of the networking and the benefits of networks
-explain the concept of communication protocols
-explain how communication occurs across a local ethernet networks
-describe access layer device and communications methods across networks
-plan, implement, and verify lokal network.

what is a network?
There are many types of networks that provide us with different kinds of services. In the course of a day, a person might make a phone call, watch a television show, listen to the radio, look up something on the Internet, or even play a video game with someone in another country. All of these activities depend on robust, reliable networks. Networks provide the ability to connect people and equipment no matter where they are in the world. People use networks without ever thinking about how they work or what it would be like if the networks did not exist.

This picture of the airport illustrates people using networks to share information, use resources and communicate with others. There are multiple types of networks shown in this scene. How many can you find?

ommunication technology in the 1990s, and before, required separate, dedicated networks for voice, video and computer data communications. Each of these networks required a different type of device in order to access the network. Telephones, televisions, and computers used specific technologies and different dedicated network structures, to communicate. But what if people want to access all of these network services at the same time, possibly using a single device?

New technologies create a new kind of network that delivers more than a single type of service. Unlike dedicated networks, these new converged networks are capable of delivering voice, video and data services over the same communication channel or network structure.

New products are coming to market that take advantage of the capabilities of converged information networks. People can now watch live video broadcasts on their computers, make a telephone call over the Internet, or search the Internet using a television. Converged networks make this possible.

In this course, the term network refers to these new multi-purpose, converged information networks.

benefits of networking
Networks come in all sizes. They can range from simple networks consisting of two computers, to networks connecting millions of devices. Networks installed in small offices, or homes and home offices, are referred to as SOHO networks. SOHO networks enable sharing of resources, such as printers, documents, pictures and music between a few local computers.

In business, large networks can be used to advertise and sell products, order supplies, and communicate with customers. Communication over a network is usually more efficient and less expensive than traditional forms of communication, such as regular mail or long distance phone calls. Networks allow for rapid communication such as email and instant messaging, and provide consolidation, storage, and access to information on network servers.

Business and SOHO networks usually provide a shared connection to the Internet. The Internet is considered a "network of networks" because it is literally made up of thousands of networks that are connected to each other.

Here are other uses of a network and the Internet:
Sharing music and video files
Research and on-line learning
Chatting with friends
Planning vacations
Purchasing gifts and supplies

Can you think of other ways people use networks and the Internet in their daily lives?
basic network components
There are many components that can be part of a network, for example personal computers, servers, networking devices, and cabling. These components can be grouped into four main categories:
Hosts
Shared peripherals
Networking devices
Networking media

The network components that people are most familiar with are hosts and shared peripherals. Hosts are devices that send and receive messages directly across the network.

Shared peripherals are not directly connected to the network, but instead are connected to hosts. The host is then responsible for sharing the peripheral across the network. Hosts have computer software configured to enable people on the network to use the attached peripheral devices.

The network devices, as well as networking media, are used to interconnect hosts.

Some devices can play more than one role, depending on how they are connected. For example, a printer directly connected to a host (local printer) is a peripheral. A printer directly connected to a network device and participates directly in network communications is a host.

3.1.4 computer roles in a network
All computers connected to a network that participate directly in network communication are classified as hosts. Hosts can send and receive messages on the network. In modern networks, computer hosts can act as a client, a server, or both. The software installed on the computer determines which role the computer plays.

Servers are hosts that have software installed that enable them to provide information, like email or web pages, to other hosts on the network. Each service requires separate server software. For example, a host requires web server software in order to provide web services to the network.

Clients are computer hosts that have software installed that enable them to request and display the information obtained from the server. An example of client software is a web browser, like Internet Explorer.

A computer with server software can provide services simultaneously to one or many clients.

Additionally, a single computer can run multiple types of server software. In a home or small business, it may be necessary for one computer to act as a file server, a web server, and an email server.

A single computer can also run multiple types of client software. There must be client software for every service required. With multiple clients installed, a host can connect to multiple servers at the same time. For example, a user can check email and view a web page while instant messaging and listening to Internet radio.

Activity
Drag the clients to multiple servers.

peer-to-peer networks
Client and server software usually runs on separate computers, but it is also possible for one computer to carry out both roles at the same time. In small businesses and homes, many computers function as the servers and clients on the network. This type of network is called a peer-to-peer network.

The simplest peer-to-peer network consists of two directly connected computers using a wired or wireless connection.

Multiple PCs can also be connected to create a larger peer-to-peer network but this requires a network device, such as a hub, to interconnect the computers.

The main disadvantage of a peer-to-peer environment is that the performance of a host can be slowed down if it is acting as both a client and a server at the same time.

In larger businesses, due to the potential for high amounts of network traffic, it is often necessary to have dedicated servers to support the number of service requests.

the advantages of peer-of-peer networking:
-easy to set up
-less complexity
-lower cost since network device and dedicated
-servers may not be required
-can be used for simple tasks such us transferring files and sharing printers
the disadvantages peer-of-peer of network
-no centralized administration
-not as secure
-not scalable
-all devices my act as both clients and servers which can slow their performance

Activity
Click the appropriate role of client, server or both.

Lab Activity
Click the lab icon to begin.

network topologies
In a simple network consisting of a few computers, it is easy to visualize how all of the various components connect. As networks grow, it is more difficult to keep track of the location of each component, and how each is connected to the network. Wired networks require lots of cabling and network devices to provide connectivity for all network hosts.

When networks are installed, a physical topology map is created to record where each host is located and how it is connected to the network. The physical topology map also shows where the wiring is installed and the locations of the networking devices that connect the hosts. Icons are used to represent the actual physical devices within the topology map. It is very important to maintain and update physical topology maps to aid future installation and troubleshooting efforts.

In addition to the physical topology map, it is sometimes necessary to also have a logical view of the network topology. A logical topology map groups hosts by how they use the network, no matter where they are physically located. Host names, addresses, group information and applications can be recorded on the logical topology map.

The graphics illustrate the difference between logical and physical topology maps.
principles of communication
source, channel, and destination
The primary purpose of any network is to provide a method to communicate information. From the very earliest primitive humans to the most advanced scientists of today, sharing information with others is crucial for human advancement.

All communication begins with a message, or information, that must be sent from one individual or device to another. The methods used to send, receive and interpret messages change over time as technology advances.

All communication methods have three elements in common. The first of these elements is the message source, or sender. Message sources are people, or electronic devices, that need to communicate a message to other individuals or devices. The second element of communication is the destination, or receiver, of the message. The destination receives the message and interprets it. A third element, called a channel, provides the pathway over which the message can travel from source to destination.

3.2.2 rules of communication
In any conversation between two people, there are many rules, or protocols, that the two must follow in order for the message to be successfully delivered and understood. Among the protocols for successful human communication are:
Identification of sender and receiver
Agreed-upon medium or channel (face-to-face, telephone, letter, photograph)
Appropriate communication mode (spoken, written, illustrated, interactive or one-way)
Common language
Grammar and sentence structure
Speed and timing of delivery

Imagine what would happen if no protocols or rules existed to govern how people communicate with each other. Would you be able to understand them? Are you able to read the paragraph that does not follow commonly accepted protocols?

rules govern communications between humans. it is very difficult to understand messages that are not correctly formatted and do not follow the established rules and protocols. the structur of grammar, the language, the punctuation and the sentence make the conviguration humanly understandable for many different individuals

source, channel and destination of the message. The rules used to communicate over one medium, like a telephone call, are not necessarily the same as communication using another medium, such as a letter.

Protocols define the details of how the message is transmitted, and delivered. This includes issues of:
Message format
Message size
Timing
Encapsulation
Encoding
Standard message pattern

Many of the concepts and rules that make human communication reliable and understandable also apply to computer communication.

message encoding
One of the first steps to sending a message is encoding it. Written words, pictures, and spoken languages each use a unique set of codes, sounds, gestures, and/or symbols to represent the thoughts being shared. Encoding is the process of converting thoughts into the language, symbols, or sounds, for transmission. Decoding reverses this process in order to interpret the thought.

Imagine a person watching a sunset and then calling someone else to talk about how beautiful the sunset looks. To communicate the message, the sender must first convert, or encode, their thoughts and perceptions about the sunset into words. The words are spoken into the telephone using the sounds and inflections of spoken language that convey the message. On the other end of the telephone line, the person listening to the description, receives and decodes the sounds in order to visualize the image of the sunset described by the sender.

Encoding also occurs in computer communication. Encoding between hosts must be in an appropriate form for the medium. Messages sent across the network are first converted into bits by the sending host. Each bit is encoded into a pattern of sounds, light waves, or electrical impulses depending on the network media over which the bits are transmitted. The destination host receives and decodes the signals in order to interpret the message.

message formating
When a message is sent from source to destination, it must use a specific format or structure. Message formats depend on the type of message and the channel that is used to deliver the message.

Letter writing is one of the most common forms of written human communication. For centuries, the agreed format for personal letters has not changed. In many cultures, a personal letter contains the following elements:
An identifier of the recipient
A salutation or greeting
The message content
A closing phrase
An identifier of the sender

In addition to having the correct format, most personal letters must also be enclosed, or encapsulated, in an envelope for delivery. The envelope has the address of the sender and receiver on it, each located at the proper place on the envelope. If the destination address and formatting are not correct, the letter is not delivered.

The process of placing one message format (the letter) inside another message format (the envelope) is called encapsulation. De-encapsulation occurs when the process is reversed by the recipient and the letter is removed from the envelope.

A letter writer uses an accepted format to ensure that the letter is delivered and understood by the recipient. In the same way, a message that is sent over a computer network follows specific format rules for it to be delivered and processed. Just as a letter is encapsulated in an envelope for delivery, so computer messages are encapsulated. Each computer message is encapsulated in a specific format, called a frame, before it is sent over the network. A frame acts like an envelope; it provides the address of the intended destination and the address of the source host.

The format and contents of a frame are determined by the type of message being sent and the channel over which it is communicated. Messages that are not correctly formatted are not successfully delivered to or processed by the destination host.

3.2.5 message sizes
Imagine what it would be like to read this course if it all appeared as one long sentence; it would not be easy to read and comprehend. When people communicate with each other, the messages that they send are usually broken into smaller parts or sentences. These sentences are limited in size to what the receiving person can process at one time. An individual conversation may be made up of many smaller sentences to ensure that each part of the message is received and understood.

Likewise, when a long message is sent from one host to another over a network, it is necessary to break the message into smaller pieces. The rules that govern the size of the pieces, or frames, communicated across the network are very strict. They can also be different, depending on the channel used. Frames that are too long or too short are not delivered.

The size restrictions of frames require the source host to break a long message into individual pieces that meet both the minimum and maximum size requirements. Each piece is encapsulated in a separate frame with the address information, and is sent over the network. At the receiving host, the messages are de-encapsulated and put back together to be processed and interpreted.

message timing
One factor that affects how well a message is received and understood is timing. People use timing to determine when to speak, how fast or slow to talk, and how long to wait for a response. These are the rules of engagement.

Access Method

Access Method determines when someone is able to send a message. These timing rules are based on the environment. For example, you may be able to speak whenever you have something to say. In this environment, a person must wait until no one else is talking before speaking. If two people talk at the same time, a collision of information occurs and it is necessary for the two to back off and start again. These rules ensure communication is successful. Likewise, it is necessary for computers to define an access method. Hosts on a network need an access method to know when to begin sending messages and how to respond when errors occur.

Flow Control

Timing also effects how much information can be sent and the speed that it can be delivered. If one person speaks too quickly, it is difficult for the other person to hear and understand the message. The receiving person must ask the sender to slow down. In network communication, a sending host can transmit messages at a faster rate than the destination host can receive and process. Source and destination hosts use flow control to negotiate correct timing for successful communication.

Response Timeout

If a person asks a question and does not hear a response within an acceptable amount of time, the person assumes that no answer is coming and reacts accordingly. The person may repeat the question, or may go on with the conversation. Hosts on the network also have rules that specify how long to wait for responses and what action to take if a response timeout occurs.

3.2.7 message pattern
Sometimes, a person wants to communicate information to a single individual. At other times, the person may need to send information to a group of people at the same time, or even to all people in the same area. A conversation between two people is an example of a one-to-one pattern of communication. When a group of recipients need to receive the same message simultaneously, a one-to-many or one-to-all message pattern is necessary.

There are also times when the sender of a message needs to be sure that the message is delivered successfully to the destination. In these cases, it is necessary for the recipient to return an acknowledgement to the sender. If no acknowledgement is required, the message pattern is referred to as unacknowledged.

Hosts on a network use similar message patterns to communicate.

A one-to-one message pattern is referred to as a unicast, meaning that there is only a single destination for the message.

When a host needs to send messages using a one-to-many pattern, it is referred to as a multicast. Multicasting is the delivery of the same message to a group of host destinations simultaneously.

If all hosts on the network need to receive the message at the same time, a broadcast is used. Broadcasting represents a one-to-all message pattern. Additionally, hosts have requirements for acknowledged versus unacknowledged messages.

3.2.8 protocol use in communication
All communication, both human and computer, is governed by pre-established rules, or protocols. These protocols are determined by the characteristics of the source, channel and destination. Based on the source, channel and destination, the protocols define the details for the issues of message format, message size, timing, encapsulation, encoding and standard message pattern.

3.3 communicating on a local wired network
3.3.1 importance of protocols
Computers, just like humans, use rules, or protocols, in order to communicate.

Protocols are especially important on a local network. In a wired environment, a local network is defined as an area where all hosts must "speak the same language" or in computer terms "share a common protocol".

If everyone in the same room spoke a different language they would not be able to communicate. Likewise, if devices in a local network did not use the same protocols they would not be able to communicate.

The most common set of protocols used on local wired networks is Ethernet.

The Ethernet protocol defines many aspects of communication over the local network, including: message format, message size, timing, encoding, and message patterns.

3.3.2. standardization of protocols
In the early days of networking, each vendor used their own, proprietary methods of interconnecting network devices and networking protocols. Equipment from one vendor could not communicate with equipment from another.

As networks became more widespread, standards were developed that defined rules by which network equipment from different vendors operated. Standards are beneficial to networking in many ways:
Facilitate design
Simplify product development
Promote competition
Provide consistent interconnections
Facilitate training
Provide more vendor choices for customers

There is no official local networking standard protocol, but over time, one technology, Ethernet, has become more common than the others. It has become a de facto standard.

The Institute of Electrical and Electronic Engineers, or IEEE (pronounced eye-triple-e), maintains the networking standards, including Ethernet and wireless standards. IEEE committees are responsible for approving and maintaining the standards for connections, media requirements and communications protocols. Each technology standard is assigned a number that refers to the committee that is responsible for approving and maintaining the standard. The committee responsible for the Ethernet standards is 802.3.

Since the creation of Ethernet in 1973, standards have evolved for specifying faster and more flexible versions of the technology. This ability for Ethernet to improve over time is one of the main reasons that it has become so popular. Each version of Ethernet has an associated standard. For example, 802.3 100BASE-T represents the 100 Megabit Ethernet using twisted pair cable standards. The standard notation translates as:
100 is the speed in Mbps
BASE stands for baseband transmission
T stands for the type of cable, in this case, twisted pair.

Early versions of Ethernet were relatively slow at 10 Mbps. The latest versions of Ethernet operate at 10 Gigabits per second and faster. Imagine how much faster these new versions are than the original Ethernet networks.

3.3.3 physical addressing
All communication requires a way to identify the source and destination. The source and destination in human communication are represented by names.

When a name is called, the person with that name listens to the message and responds. Other people in the room may hear the message, but they ignore it because it is not addressed to them.

On Ethernet networks, a similar method exists for identifying source and destination hosts. Each host connected to an Ethernet network is assigned a physical address which serves to identify the host on the network.

Every Ethernet network interface has a physical address assigned to it when it is manufactured. This address is known as the Media Access Control (MAC) Address. The MAC address identifies each source and destination host on the network.

Ethernet networks are cable based, meaning that a copper or fiber optic cable connects hosts and networking devices. This is the channel used for communications between the hosts.

When a host on an Ethernet network communicates, it sends frames containing its own MAC address as the source and the MAC address of the intended recipient. Any hosts that receive the frame will decode the frame and read the destination MAC address. If the destination MAC address matches the address configured on the NIC, it will process the message and store it for the host application to use. If the destination MAC address does not match the host MAC address, the NIC will ignore the message.

3.3.4 ethernet communication
The Ethernet protocol standards define many aspects of network communication including frame format, frame size, timing and encoding.

When messages are sent between hosts on an Ethernet network, the hosts format the messages into the frame layout that is specified by the standards. Frames are also referred to as Protocol Data Units (PDUs).

The format for Ethernet frames specifies the location of the destination and source MAC addresses, and additional information including:
Preamble for sequencing and timing
Start of frame delimiter
Length and type of frame
Frame check sequence to detect transmission errors.

The size of Ethernet frames is limited to a maximum of 1518 bytes and a minimum size of 64 bytes. Frames that do not match these limits are not processed by the receiving hosts. In addition to the frame formats, sizes and timing, Ethernet standards define how the bits making up the frames are encoded onto the channel. Bits are transmitted as either electrical impulses over copper cable or as light impulses over fiber optic cable.

3.3.5 hierarchical design of ethernet networks
Imagine how difficult communication would be if the only way to send a message to someone was to use the person's name. If there were no street addresses, cities, towns, or country boundaries, delivering a message to a specific person across the world would be nearly impossible.

On an Ethernet network, the host MAC address is similar to a person's name. A MAC address indicates the individual identity of a specific host, but it does not indicate where on the network the host is located. If all hosts on the Internet (over 400 million of them) were each identified by only their unique MAC address, imagine how difficult it would be to locate a single one.

Additionally, Ethernet technology generates a large amount of broadcast traffic in order for hosts to communicate. Broadcasts are sent to all hosts within a single network. Broadcasts consume bandwidth and slow network performance. What would happen if the millions of hosts attached to the Internet were all in one Ethernet network and were using broadcasts?

For these two reasons, large Ethernet networks consisting of many hosts are not efficient. It is better to divide larger networks into smaller, more manageable pieces. One way to divide larger networks is to use a hierarchical design model.

In networking, hierarchical design is used to group devices into multiple networks that are organized in a layered approach. It consists of smaller, more manageable groups that allow local traffic to remain local. Only traffic that is destined for other networks is moved to a higher layer.

A hierarchical, layered design provides increased efficiency, optimization of function, and increased speed. It allows the network to scale as required because additional local networks can be added without impacting the performance of the existing ones.

The hierarchical design has three basic layers:
Access Layer - to provide connections to hosts in a local Ethernet network.
Distribution Layer - to interconnect the smaller local networks.
Core Layer - a high-speed connection between distribution layer devices.

With this new hierarchical design, there is a need for a logical addressing scheme that can identify the location of a host. This is the Internet Protocol (IP) addressing scheme.

logical addressing
A person's name usually does not change. A person's address on the other hand, relates to where they live and can change. On a host, the MAC address does not change; it is physically assigned to the host NIC and is known as the physical address. The physical address remains the same regardless of where the host is placed on the network.

The IP address is similar to the address of a person. It is known as a logical address because it is assigned logically based on where the host is located. The IP address, or network address, is assigned to each host by a network administrator based on the local network.

IP addresses contain two parts. One part identifies the local network. The network portion of the IP address will be the same for all hosts connected to the same local network. The second part of the IP address identifies the individual host. Within the same local network, the host portion of the IP address is unique to each host.

Both the physical MAC and logical IP addresses are required for a computer to communicate on a hierarchical network, just like both the name and address of a person are required to send a letter.

access and distribution layers and devices
IP traffic is managed based on the characteristics and devices associated with each of the three layers: Access, Distribution and Core. The IP address is used to determine if traffic should remain local or be moved up through the layers of the hierarchical network.

Access Layer

The Access Layer provides a connection point for end user devices to the network and allows multiple hosts to connect to other hosts through a network device, usually a hub or switch. Typically, all devices within a single Access Layer will have the same network portion of the IP address.

If a message is destined for a local host, based on the network portion of the IP address, the message remains local. If it is destined for a different network, it is passed up to the Distribution Layer. Hubs and switches provide the connection to the Distribution Layer devices, usually a router.

Distribution Layer

The Distribution Layer provides a connection point for separate networks and controls the flow of information between the networks. It typically contains more powerful switches than the Access Layer as well as routers for routing between networks. Distribution Layer devices control the type and amount of traffic that flows from the Access Layer to the Core Layer.

Core Layer

The Core Layer is a high-speed backbone layer with redundant (backup) connections. It is responsible for transporting large amounts of data between multiple end networks. Core Layer devices typically include very powerful, high-speed switches and routers. The main goal of the Core Layer.

Core Layer

The Core Layer is a high-speed backbone layer with redundant (backup) connections. It is responsible for transporting large amounts of data between multiple end networks. Core Layer devices typically include very powerful, high-speed switches and routers. The main goal of the Core Layer is to transport data quickly. Hubs, switches, and routers are discussed in more detail in the next two sections.

building the access layer of an ethernet network
access layers
The Access Layer is the most basic level of the network. It is the part of the network in which people gain access to other hosts and to shared files and printers. The Access Layer is composed of host devices, as well as the first line of networking devices to which they are attached.

Networking devices enable us to connect many hosts with each other and also provide those hosts access to services offered over the network. Unlike the simple network consisting of two hosts connected by a single cable, in the Access Layer, each host is connected to a networking device. This type of connectivity is shown in the graphic.

Within an Ethernet network, each host is able to connect directly to an Access Layer networking device using a point-to-point cable. These cables are manufactured to meet specific Ethernet standards. Each cable is plugged into a host NIC and then into a port on the networking device. There are several types of networking devices that can be used to connect hosts at the Access Layer, including Ethernet hubs and switches.

function of hubs
A hub is one type of networking device that is installed at the Access Layer of an Ethernet network. Hubs contain multiple ports that are used to connect hosts to the network. Hubs are simple devices that do not have the necessary electronics to decode the messages sent between hosts on the network. Hubs cannot determine which host should get any particular message. A hub simply accepts electronic signals from one port and regenerates (or repeats) the same message out all of the other ports.

Remember that the NIC on a host accepts messages only addressed to the correct MAC address. Hosts ignore messages that are not addressed to them. Only the host specified in the destination address of the message processes the message and responds to the sender.

All of the ports on the Ethernet hub connect to the same channel to send and receive messages. Because all hosts must share the bandwidth available on that channel, a hub is referred to as a shared-bandwidth device.

Only one message can be sent through an Ethernet hub at a time. It is possible for two or more hosts connected to a hub to attempt to send a message at the same time. If this happens, the electronic signals that make up the messages collide with each other at the hub.

A collision causes the messages to become garbled and unreadable by the hosts. A hub does not decode the messages; therefore it does not detect that the message is garbled and repeats it out all the ports. The area of the network where a host can receive a garbled message resulting from a collision is known as a collision domain.

Inside a collision domain, when a host receives a garbled message, it detects that a collision has occurred. Each sending host waits a short amount of time and then attempts to send, or retransmit, the message again. As the number of hosts connected to the hub increases, so does the chance of collisions. More collisions cause more retransmissions. Excessive retransmissions can clog up the network and slow down network traffic. For this reason, it is necessary to limit the size of a collision domain.

3.4.3 fungtion of switches
An Ethernet switch is a device that is used at the Access Layer. Like a hub, a switch connects multiple hosts to the network. Unlike a hub, a switch can forward a message to a specific host. When a host sends a message to another host on the switch, the switch accepts and decodes the frames to read the physical (MAC) address portion of the message.

A table on the switch, called a MAC address table, contains a list of all of the active ports and the host MAC addresses that are attached to them. When a message is sent between hosts, the switch checks to see if the destination MAC address is in the table. If it is, the switch builds a temporary connection, called a circuit, between the source and destination ports. This new circuit provides a dedicated channel over which the two hosts can communicate. Other hosts attached to the switch do not share bandwidth on this channel and do not receive messages that are not addressed to them. A new circuit is built for every new conversation between hosts. These separate circuits allow many conversations to take place at the same time, without collisions occurring.

What happens when the switch receives a frame addressed to a new host that is not yet in the MAC address table? If the destination MAC address is not in the table, the switch does not have the necessary information to create an individual circuit. When the switch cannot determine where the destination host is located, it uses a process called flooding to forward the message out to all attached hosts. Each host compares the destination MAC address in the message to its own MAC address, but only the host with the correct destination address processes the message and responds to the sender.

How does the MAC address of a new host get into the MAC address table? A switch builds the MAC address table by examining the source MAC address of each frame that is sent between hosts. When a new host sends a message or responds to a flooded message, the switch immediately learns its MAC address and the port to which it is connected. The table is dynamically updated each time a new source MAC address is read by the switch. In this way, a switch quickly learns the MAC addresses of all attached hosts.

Sometimes, it is necessary to connect another networking device, like a hub, to a switch port. This is done to increase the number of hosts that can be connected to the network. When a hub is connected to a switch port, the switch associates the MAC addresses of all hosts connected to that hub with the single port on the switch. Occasionally, one host on the attached hub sends a message to another host attached to the same hub. In this case, the switch receives the frame and checks the table to see where the destination host is located. If both the source and destination hosts are located on the same port, the switch discards the message.

When a hub is connected to a switch port, collisions can occur on the hub. The hub forwards to all ports the damaged messages resulting from a collision. The switch receives the garbled message, but, unlike a hub, a switch does not forward the damaged messages caused by collisions. As a result, every switch port creates a separate collision domain. This is a good thing. The fewer hosts contained in a collision domain, the less likely it is that a collision will occur.

3.4.4 broadcast messaging
When hosts are connected using either a hub or a switch, a single local network is created. Within the local network it is often necessary for one host to be able to send messages to all the other hosts at the same time. This can be done using a message known as a broadcast. Broadcasts are useful when a host needs to find information without knowing exactly what other host can supply it or when a host wants to provide information to all other hosts in the same network in a timely manner.

A message can only contain one destination MAC address. So, how is it possible for a host to contact every other host on the local network without sending out a separate message to each individual MAC?

To solve this problem, broadcast messages are sent to a unique MAC address that is recognized by all hosts. The broadcast MAC address is actually a 48-bit address made up of all ones. Because of their length, MAC addresses are usually represented in hexadecimal notation. The broadcast MAC address in hexadecimal notation is FFFF.FFFF.FFFF. Each F in the hexadecimal notation represents four ones in the binary address.

When a host receives a message addressed to the broadcast address, it accepts and processes the message as though the message was addressed directly to it. When a host sends a broadcast message, hubs and switches forward the message to every connected host within the same local network. For this reason, a local network is also referred to as a broadcast domain.

If too many hosts are connected to the same broadcast domain, broadcast traffic can become excessive. The number of hosts and the amount of network traffic that can be supported on the local network is limited by the capabilities of the hubs and switches used to connect them. As the network grows and more hosts are added, network traffic, including broadcast traffic, increases. It is often necessary to divide one local network, or broadcast domain, into multiple networks to improve performance.

3.4.6 MAC and IP
On a local Ethernet network, a NIC only accepts a frame if the destination address is either the broadcast MAC address, or else corresponds to the MAC address of the NIC.

Most network applications, however, rely on the logical destination IP address to identify the location of the servers and clients.

What if a sending host only has the logical IP address of the destination host? How does the sending host determine what destination MAC address to place within the frame?

The sending host can use an IP protocol called address resolution protocol (ARP) to discover the MAC address of any host on the same local network.

3.4.7 address resolution protocols (ARP)
ARP uses a three step process to discover and store the MAC address of a host on the local network when only the IP address of the host is known.

1. The sending host creates and sends a frame addressed to a broadcast MAC address. Contained in the frame is a message with the IP address of the intended destination host.

2. Each host on the network receives the broadcast frame and compares the IP address inside the message with its configured IP address. The host with the matching IP address sends its MAC address back to the original sending host.

3. The sending host receives the message and stores the MAC address and IP address information in a table called an ARP table.

Once the sending host has the MAC address of the destination host in its ARP table, it can send frames directly to the destination without doing an ARP request.

building the distribution layer of a network
distribution layer
As networks grow, it is often necessary to divide one local network into multiple Access Layer networks. There are many ways to divide networks based on different criteria, including:
Physical location
Logical function
Security requirements
Application requirements

The Distribution Layer connects these independent local networks and controls the traffic flowing between them. It is responsible for ensuring that traffic between hosts on the local network stays local. Only traffic that is destined for other networks is passed on. The Distribution Layer can also filter incoming and outgoing traffic for security and traffic management.

Networking devices that make up the Distribution Layer are designed to interconnect networks, not individual hosts. Individual hosts are connected to the network via Access Layer devices, such as hubs and switches. The Access Layer devices are connected to each other via the Distribution Layer device, such as routers.

function of routers
A router is a networking device that connects a local network to other local networks. At the Distribution Layer of the network, routers direct traffic and perform other functions critical to efficient network operation. Routers, like switches, are able to decode and read the messages that are sent to them. Unlike switches, which only decode (unencapsulate) the frame containing the MAC address information, routers decode the packet that is encapsulated within the frame.

The packet format contains the IP addresses of the destination and source hosts, as well as the message data being sent between them. The router reads the network portion of the destination IP address and uses it to find which one of the attached networks is the best way to forward the message to the destination.

Anytime the network portion of the IP addresses of the source and destination hosts do not match, a router must be used to forward the message. If a host located on network 1.1.1.0 needs to send a message to a host on network 5.5.5.0, the host will forward the message to the router. The router receives the message and unencapsulates it to read the destination IP address. It then determines where to forward the message. It re-encapsulates the packet back into a frame, and forwards the frame on to its destination.

How does the router determine what path to send the message to get to the destination network?

Each port, or interface, on a router connects to a different local network. Every router contains a table of all locally-connected networks and the interfaces that connect to them. These routing tables can also contain information about the routes, or paths, that the router uses to reach other remote networks that are not locally attached.

When a router receives a frame, it decodes the frame to get to the packet containing the destination IP address. It matches the address of the destination to all of the networks that are contained in the routing table. If the destination network address is in the table, the router encapsulates the packet in a new frame in order to send it out. It forwards the new frame out of the interface associated with the path, to the destination network. The process of forwarding the packets toward their destination network is called routing.

Router interfaces do not forward messages that are addressed to the broadcast MAC address. As a result, local network broadcasts are not sent across routers to other local networks.

default gateway
The method that a host uses to send messages to a destination on a remote network differs from the way a host sends messages on the same local network. When a host needs to send a message to another host located on the same network, it will forward the message directly. A host will use ARP to discover the MAC address of the destination host. It includes the destination IP address within the packet and encapsulates the packet into a frame containing the MAC address of the destination and forwards it out.

On the other hand, when a host needs to send a message to a remote network, it must use the router. The host includes the IP address of the destination host within the packet just like before. However, when it encapsulates the packet into a frame, it uses the MAC address of the router as the destination for the frame. In this way, the router will receive and accept the frame based on the MAC address.

How does the source host determine the MAC address of the router? A host is given the IP address of the router through the default gateway address configured in its TCP/IP settings. The default gateway address is the address of the router interface connected to the same local network as the source host. All hosts on the local network use the default gateway address to send messages to the router. Once the host knows the default gateway IP address, it can use ARP to determine the MAC address. The MAC address of the router is then placed in the frame, destined for another network.

It is important that the correct default gateway be configured on each host on the local network. If no default gateway is configured in the host TCP/IP settings, or if the wrong default gateway is specified, messages addressed to hosts on remote networks cannot be delivered.

tables maintained by routers
Routers move information between local and remote networks. To do this, routers must use both ARP and routing tables to store information. Routing tables are not concerned with the addresses of individual hosts. Routing tables contain the addresses of networks and the best path to reach those networks. Entries can be made to the routing table in two ways: dynamically updated by information received from other routers in the network, or manually entered by a network administrator. Routers use the routing tables to determine which interface to use to forward a message to its intended destination.

If the router cannot determine where to forward a message, it will drop it. Network administrators configure a routing table with a default route to keep a packet from being dropped because the path to the destination network is not in the routing table. A default route is the interface through which the router forwards a packet containing an unknown destination IP network address. This default route usually connects to another router that can forward the packet towards its final destination network.

A router forwards a frame to one of two places: a directly connected network containing the actual destination host, or to another router on the path to reach the destination host. When a router encapsulates the frame to forward it out of an Ethernet interface, it must include a destination MAC address.

This is the MAC address of the actual destination host, if the destination host is part of a network locally connected to the router. If the router must forward the packet to another router, it will use the MAC address of the connected router. Routers obtain these MAC addresses from ARP tables.

Each router interface is part of the local network to which it is attached and maintains its own ARP table for that network. The ARP tables contain the MAC addresses and IP addresses of all of the individual hosts on that network.

local area network (LAN)
The term Local Area Network (LAN) refers to a local network, or a group of interconnected local networks that are under the same administrative control. In the early days of networking, LANs were defined as small networks that existed in a single physical location. While LANs can be a single local network installed in a home or small office, the definition of LAN has evolved to include interconnected local networks consisting of many hundreds of hosts, installed in multiple buildings and locations.

The important thing to remember is that all of the local networks within a LAN are under one administrative control. Other common characteristics of LANs are that they typically use Ethernet or wireless protocols, and they support high data rates.

The term Intranet is often used to refer to a private LAN that belongs to an organization, and is designed to be accessible only by the organization's members, employees, or others with authorization.

adding hosts to local and remote networks
Within a LAN, it is possible to place all hosts on a single local network or divide them up between multiple networks connected by a Distribution Layer. The answer depends on desired results. Placing all hosts on a single local network allows them to be seen by all other hosts. This is because there is one broadcast domain and hosts use ARP to find each other.

In a simple network design it may be beneficial to keep all hosts within a single local network. However, as networks grow in size, increased traffic will decrease network performance and speed. In this case, it may be beneficial to move some hosts onto a remote network.

Placing additional hosts on a remote network will decrease the impact of traffic demands. However, hosts on one network will not be able to communicate with hosts on the other without the use of routing. Routers increase the complexity of the network configuration and can introduce latency, or time delay, on packets sent from one local network to the other.

learn to use the packet tracer
packet tracer is a graphical learning and simulation tool cisco developed to help program and understand how networks function. it enables you to build network topologies and test them by sending packet between devices and observing the interactions of protocols use.

plan and connect a local network
plan and document an ethernet network
Most local networks are based on Ethernet technology. This technology is both fast and efficient when used in a properly designed and constructed network. The key to installing a good network is planning before the network is actually built.

A network plan starts with the gathering of information about how the network will be used. This information includes:
The number and type of hosts to be connected to network
The applications to be used
Sharing and Internet connectivity requirements
Security and privacy considerations
Reliability and uptime expectations
Connectivity requirements including, wired and wireless

There are many considerations that must be taken into account when planning for a network installation. The logical and physical topology maps of the network need to be designed and documented before the networking equipment is purchased and the hosts are connected. Some things to consider include:

Physical environment where the network will be installed:
Temperature control: all devices have specific ranges of temperature and humidity requirements for proper operation
Availability and placement of power outlets

Physical configuration of the network:
Physical location of devices such as routers, switches, and hosts
How all devices are interconnected
Location and length of all cable runs
Hardware configuration of end devices such as hosts and servers

Logical configuration of the network:
Location and size of broadcast and collision domains
IP addressing scheme
Naming scheme
Sharing configuration
Permissions

3.6.2 prototypes
Once the network requirements are documented, and the physical and logical topology maps created, the next step in the implementation process is to test the network design. One of the ways to test a network design is to create a working model, or prototype, of the network.

Prototyping is essential as networks grow in size and complexity. A prototype allows a network administrator to test whether or not the planned network will operate as expected, before money is spent on equipment and installation. Documentation should be maintained on all aspects of the prototyping process.

Various tools and techniques are available for network prototyping; this includes real equipment set up in a lab environment, modeling and simulation tools. Packet Tracer is one example of a simulation and modeling tool that can be used for prototyping.

multi-function device
Most home and small business networks do not require high-volume devices used in large business environments; smaller scale devices may well be suitable. However, the same functionality of routing and switching is required. This need has led to the development of products that have the functionality of multiple network devices, such as a router with switching functionality and a wireless access point. For the purpose of this course, multi-function devices will be referred to as integrated routers. Integrated routers can range from small devices designed for home office and small business applications to more powerful devices that can support enterprise branch offices.

An integrated router is like having several different devices connected together. For example, the connection between the switch and the router still occurs, but it occurs internally. When a broadcast is received on a switch port, the integrated router forwards the broadcast to all ports including the internal router connection. The router portion of the integrated router stops the broadcasts from going any further.

There are low-cost multi-function devices available for home and small business networks that offer integrated routing, switching, wireless and security capabilities. An example of this type of integrated router is a Linksys wireless router. They are simple in design and do not typically have separate components. In the event of a failure, it is not possible to replace any single failed component. As such, they create a single point of failure, and are not optimized for any one function.

Another example of an integrated router is the Cisco integrated services router or ISR. The Cisco ISR product family offers a wide range of products, including those designed for small office and home office environments as well as those designed for larger networks. Many of the ISRs offer modularity and have separate components for each function, such as a switch component and a router component. This enables individual components to be added, replaced and upgraded as necessary.

connecting the linksys router
front view :
the linksys is a simplifield, low-cost devices that carries out the functionality of multiple network device (switch, router, wireless access point)
light emitting diodes (LEDs) indicate the connection status of each port

All devices connected to the switch ports should be in the same broadcast domain. This means that all devices must have an IP address from the same network. Any device that has a different network portion within the IP address will not be able to communicate.

Additionally, Microsoft Windows makes use of computer names to identify other devices on the network. It is important to use these names as well as all IP address information in the planning and documentation to assist in future troubleshooting.

To display the current IP configuration in Microsoft Windows, use the command ipconfig. More detailed information, including host name, is available with the ipconfig /all. Document all information from the connection and configuration process.

Once hosts are communicating across the network, it is important to document network performance. This is known as determining the baseline for the network, and is used as an indication of normal operations. When comparing future network performance with the baseline, it can indicate if possible issues exist.

3.6.5 sharing resources
One of the most common purposes of networking is to share resources such as files and printers. Windows XP enables remote users to access a local machine and its resources through Sharing. It is important to consider security issues, and to assign specific permissions to shared resources.

By default, Windows XP uses a process known as Simple File Sharing. With Simple File Sharing, specific users and groups cannot be prevented from accessing shared files.

Simple File Sharing can be disabled so that more specific security access levels can be assigned. When this is done, the following permissions are available to assign to resources:
Full Control
Modify
Read & Execute
List Folder Contents
Read
Write

When a user accesses a file on a remote device, Windows Explorer allows the user to map a drive to a remote folder or resource. This maps a specific drive letter, for example M:, to the remote resource. This enables the user to treat the resource as though it was locally connected.

chapter summary
summary

this chapter discussed basic concepts and benefits of networking, and the characteristics of local ethernet network.
-information networks can carry voice, video and data.
-information network consist of peripherals, hosts, network devices and medis.
-topology diagrams used to depict both logical and physical network design.
-hosts can play the role of client or server or both.

-all communication has a source, a destination and a channel.
-computer communications operate under special rules, called protocols.
-protocols define the characteristics of a message including: encoding, formating, encapsulation, size, timing and patterns.
-to communicate on a local network requires the computers share a common protocol.
-the most common protocol used on local wired networks is industry-standart enthernet
-each local host in an ethernet network is identified by a physical MAC adress, which is configured into a host's NIC.

it is common to divide large network into smaller, more manageable, ones using a layered hierarchical design with can include the following layers:
-access
-distribution
-core
each of these layers has a primari function and associated devices.

-logical IP addresses are used to identify the location of a host within this hierarchical design.
-to deliver a packet to an individual host requires both a phycical MAC address and logical IP address.
-ARP is use to resolve an IP address to a MAC address to local delivery.

access layer:
-the access layer is the first point of entry into the network for all hosts.
-hosts are usualy directly connected using ethernet cables to an access layer devices, suck as a hub or switch.
-MAC address and IP addresses are used on local network at the access layer.

distribution layer:
-the distribution layer connects independent local network and controls traffic between them.
-individual hosts are not usually connected directly to the distribution layer devices.
-router are them main networking devices within the distribution layer and usen IP addresses to move packets between network.

a network plan starts with the gathering of information about how the network will be used. this information includes:
-the number and type of hosts to be connected to network.
-the application to be used
-sharing and internet connectivity requirement
-security and privacy considerations
-reliability and uptime expectations
-connectivity requirements including, wired and wireless.

information gathering:
NUMBER AND TYPE OF HOSTS- where are the end user located? what type of hardware are they using? where are the servers, printers and other network devices located?
APPLICATIONS- what type of applications are running on the network?
DATA AND DEVICES TO BE SHARED- who requires access to which files and network resources such as printers?
BANDWIDTH REQUIREMENTS (SPEED)- what is an acceptable speed for the end user? do all user require the same throughput? what effect will the applications have on the throughput?
SECURITY- is the data being moved on the network of a personal or sensitive nature? could unauthorized access to this information cause harm to anyone?
RELIABILITY- how important is the network? does it need to be available 100% of the time ( this is known uptime)? how much down time can be tolerated?
REQUIREMENT FOR WIRELESS- do any or all of the end user require wireless connectivity?

-cisco ISRs, and other multi-fucntion networking devices, connect home and small business network in order for multiple hosts and share resources and to connect to the internet.
-a home internet devices is a simplified low-cost device commonly used small networks.
-these devices typically provide the functionality of a switch, router and wireless access point in one device.


Source : CCNA Discovery

Countable nouns are easy to recognize. They are things that we can count. For example: "pen". We can count pens. We can have one, two, three or more pens. Here are some more countable nouns:

* dog, cat, animal, man, person
* bottle, box, litre
* coin, note, dollar
* cup, plate, fork
* table, chair, suitcase, bag

Countable nouns can be singular or plural:

* My dog is playing.
* My dogs are hungry.

We can use the indefinite article a/an with countable nouns:

* A dog is an animal.

When a countable noun is singular, we must use a word like a/the/my/this with it:

* I want an orange. (not I want orange.)
* Where is my bottle? (not Where is bottle?)

When a countable noun is plural, we can use it alone:

* I like oranges.
* Bottles can break.

We can use some and any with countable nouns:

* I've got some dollars.
* Have you got any pens?

We can use a few and many with countable nouns:

* I've got a few dollars.
* I haven't got many pens.

What Are Nouns?

The simple definition is: a person, place or thing. Here are some examples:

* person: man, woman, teacher, John, Mary
* place: home, office, town, countryside, America
* thing: table, car, banana, money, music, love, dog, monkey

The problem with this definition is that it does not explain why "love" is a noun but can also be a verb.

Another (more complicated) way of recognizing a noun is by its:

1. Ending
2. Position
3. Function


1. Noun Ending

There are certain word endings that show that a word is a noun, for example:

* -ity > nationality
* -ment > appointment
* -ness > happiness
* -ation > relation
* -hood > childhood

But this is not true for the word endings of all nouns. For example, the noun "spoonful" ends in -ful, but the adjective "careful" also ends in -ful.

2. Position in Sentence

We can often recognise a noun by its position in the sentence.

Nouns often come after a determiner (a determiner is a word like a, an, the, this, my, such):

* a relief
* an afternoon
* the doctor
* this word
* my house
* such stupidity

Nouns often come after one or more adjectives:

* a great relief
* a peaceful afternoon
* the tall, Indian doctor
* this difficult word
* my brown and white house
* such crass stupidity

3. Function in a Sentence

Nouns have certain functions (jobs) in a sentence, for example:

* subject of verb: Doctors work hard.
* object of verb: He likes coffee.
* subject and object of verb: Teachers teach students.

But the subject or object of a sentence is not always a noun. It could be a pronoun or a phrase. In the sentence "My doctor works hard", the noun is "doctor" but the subject is "My doctor".


Source : http://www.englishclub.com

Adverbs of Frequency answer the question "How often?" or "How frequently?" They tell us how often somebody does something.

Adverbs of frequency come before the main verb (except the main verb "to be"):

100% always
usually
frequently
often

Example:
* We usually go shopping on Saturday.
* I have often done that.
* She is always late.

Occasionally, sometimes, often, frequently and usually can also go at the beginning or end of a sentence:

50% sometimes
occasionally
rarely
seldom
hardly ever

Example:
* Sometimes they come and stay with us.
* I play tennis occasionally.

Rarely and seldom can also go at the end of a sentence (often with "very"):

0% never

Example :

* We see them rarely.
* John eats meat very seldom.

An adverb is a word that tells us more about a verb. An adverb "qualifies" or "modifies" a verb (The man ran quickly). But adverbs can also modify adjectives (Tara is really beautiful), or even other adverbs (It works very well).

Many different kinds of word are called adverbs. We can usually recognise an adverb by its:

1. Function (Job)
2. Form
3. Position

1. Function

The principal job of an adverb is to modify (give more information about) verbs, adjectives and other adverbs. In the following examples, the adverb is in bold and the word that it modifies is in italics.

* Modify a verb:
- John speaks loudly. (How does John speak?)
- Mary lives locally. (Where does Mary live?)
- She never smokes. (When does she smoke?)



* Modify an adjective:
- He is really handsome.

* Modify another adverb:
- She drives incredibly slowly.

But adverbs have other functions, too. They can:

* Modify a whole sentence:
- Obviously, I can't know everything.

* Modify a prepositional phrase:
- It's immediately inside the door.

2. Form

Many adverbs end in -ly. We form such adverbs by adding -ly to the adjective. Here are some examples:

* quickly, softly, strongly, honestly, interestingly

But not all words that end in -ly are adverbs. "Friendly", for example, is an adjective.

Some adverbs have no particular form, for example:

* well, fast, very, never, always, often, still

3. Position

Adverbs have three main positions in the sentence:

* Front (before the subject):
- Now we will study adverbs.

* Middle (between the subject and the main verb):
- We often study adverbs.

* End (after the verb or object):
- We study adverbs carefully.