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Concepts of Computer Networking



At its most elementary level, a computer network consists of two computers connected to each other by a cable that allows them to share data. All computer networking, no matter how sophisticated stems from that simple system. While the idea of connecting two computers by a cable may not seem extraordinary, inretrospect it has proven to be a major achievement in communications.

Computer networking arose as an answer to the need to share data in a timely fashion. Personal computers are powerful tools that can process and manipulate large amounts of data quickly, but they do not allow users to share that data efficiently. Before networks, users needed either to print out documents or copy document files to a disk for others to edit or use them. If others made changes tothe document, there was no easy way to merge the changes. This was, and still is, known as “working in a stand-alone environment.”


Computer networks can be categorized in the following types.


A local area network (LAN) supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home. A LAN is useful for sharing resources like files, printers, games or other applications. A LAN in turn often connects to other LANs, and to the Internet . The most common type of local area network is an Ethernet LAN. The smallest home LAN can have exactly two computers; a large LAN can accommodate many thousands of computers. Many LANs are divided into logical groups called subnets.


MAN stands for metropolitan area network .It is a network of devices within an area of one to ten kilometers or with in a city. It may be a single network such as a cable television network or it may be a means of connecting a number of LANs into a larger network so that resources may be shared LAN to LAN as well as device to device.


A WAN stand for wide area network .It is spread through out the world. A WAN that is wholly owned and used by a single company is often referred to as an enterprise network. It can connect computers and other devices on opposite sides of the world. A WAN is made up of a number of interconnected LANs. Perhaps the ultimate WAN is the Internet.


Anintranetis a privatecomputer networkthat usesInternet Protocoltechnologies to securely share any part of an organization’s information or operational systems within that organization. The term is used in contrast tointernet, a network between organizations, and instead refers to a network within an organization. Sometimes the term refers only to the organization’s internalwebsite, but may be a more extensive part of the organization’s information technology infrastructure. It may host multiple private websites and constitute an important component and focal point of internal communication and collaboration.


Anextranetis a private network that usesInternet protocols,networkconnectivity. An extranet can be viewed as part of a company’sintranetthat is extended to users outside the company, usually via theInternet. It has also been described as a “state of mind” in which the Internet is perceived as a way to do business with a selected set of other companies (business-to-business, B2B), in isolation from all other Internet users. In contrast,business-to-consumer(B2C) models involve known servers of one or more companies, communicating with previously unknown consumer users.


An Internetwork is a collection of two or more LANs connected by WANs. Internworks are referred to interchangeably as data networks or simply networks. The most popular internetwork is the Internet which is open to public.


A data communication system has two main components:-


Devices and media are the physical elements or hardware of the network Hradware is often the visible components of the network platform such as a laptop, a PC or swtich etc used to connect the devices. Ocassionally some components might not be so visible.

    • DEVICES:

Devices of the network can be of two types that are the end devices and the intermediary devices, we explain both the types:-


An end use device refers to a piece of equipment that is either the ousce or the destination of a message on a network. Network users usaully only see or touch an end device, which is most often a computer. Another can generic term for an end device that sends or receives messages is a host. E.g host and end devices are Printers, Computers, Scanners, Webcams etc.


Intermediary devices connect the indivisual hosts to the network or can connect multiple networks to form an internetwork. Intermediary devices are not all the same. Some work inside the LAN to perfom switching functions and others help route messages between networks. Example of intermediary devices are Switches, Hubs and Routers etc.


Communication across a network is carried on a medium. The medium provides the channel over which the message travels from source to destination. The three main types of media in use in a network are:

    • COPPER:

A twisted pair cable usually used as a medium inside a LAN environment.


Made up of glass or plastic fibers in a vinyl coating usually used for long runs in a LAN and as a trunk.


It connects local users through air using electromagnetic waves.


Software components can be divided in to two parts, services and processes.


A network service provide information in responce to a request. Services include many of the common netowrk applications people use every day, like e-mail hosting services and web hosting services. For an instance we can take example of YAHOO enterprise, they provide mail services as well as web services, there are a number of companies offering these kind of services.


Processes provide the funtionality that directs and moves the messages through the network. Processes are less obvious to us ut are critical to the opeation of networks. For example viewing a webpage invokes one network process, clicking on a hyperlink causes a web browser to communicate with a web server, in the same way many network processes can take place at the same time.


Topology of a network is the geometrical representation of the relationship of all the links and linking devices to one another.


There are four basic physical topologies possible mesh, star, bus, and ring.


In a mesh topology every device has a dedicated point to point connection to every other device .A fully connected mesh network therefore has n(n-1)/2 physical channels to link n devices .


In star topology each device has a dedicated point to point connection only to a central controller usually called a hub . The devices are not directly connected to each other .Unlike a mesh topology ,a star topology does not allow direct traffic between devices the controller acts as an exchange : if one device wants to send data to another it sends the data to the controller which then relays the data to the other connected device.


A bus topology on the other hand is multi point one long cable acts as a back bone to link all the devices in a network nodes are connected to the bus cable by drop lines and taps a drop line is a connection running between the devices and the main cable a tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core.


In a ring topology each device has a dedicated point to point connection only with the two devices on either side of it . A signal is passed along the ring in one direction from device to device until it reaches to its destination protocols.


The Logical topology defines how the systems communicate across the physical topologies. There are two main types of logical topologies:


In a shared media topology, all the systems have the ability to access the physical layout whenever they need it. The main advantage in a shared media topology is that the systems have unrestricted access to the physical media. Of course, the main disadvantage to this topology is collisions. If two systems send information out on the wire at the same time, the packets collide and kill both packets. Ethernet is an example of a shared media topology.


The token-based topology works by using a token to provide access to the physical media. In a token-based network, there is a token that travels around the network. When a system needs to send out packets, it grabs the token off of the wire, attaches it to the packets that are sent, and sends it back out on the wire. As the token travels around the network, each system examines the token. When the packets arrive at the destination systems, those systems copy the information off of the wire and the token continues its journey until it gets back to the sender. When the sender receives the token back, it pulls the token off of the wire and sends out a new empty token to be used by the next machine.


In information technology, a protocol (from the Greek protocollon, which was a leaf of paper glued to a manuscript volume, describing its contents) is the special set of rules that end points in a telecommunication connection use when they communicate. Protocols exist at several levels in a telecommunication connection. For example, there are protocols for the data interchange at the hardware device level and protocols for data interchange at the application program level. In the standard model known as Open Systems Interconnection (OSI), there are one or more protocols at each layer in the telecommunication exchange that both ends of the exchange must recognize and observe. Protocols are often described in an industry or international standard.


For devices to communicate over the network, they must follow different protocols that perform the many tasks to be completed. The protocols define the following:

  • The format of the message
  • The way intermediary dvices share information about the path to the destination
  • The method to handle update messages between intermediary devices
  • The process to initiate and terminate communications between hosts


Interaction between protocols can be clearly understood by a simple example, the way that a web server and a web client interacts. HTTP defines the formatting and content of the requests and responses exchanged between the client and server. Both the client and server implements HTTP as part of the application. The HTTP protocol relies on other protocols to govern how the message are transported between the client and server. TCP is the transport protocol that divides the HTTP messages in to smaller pieces to be sent to the destination client, it is also responsible for controlling the size and rate at which messages are exchanged between the client and the server. Another protocol called IP is responsible for taking the formatted segments from TCP, encapsulating them into packets, assigning the appropriate addresses and selecting the best path to the destination host.


Protocols that guide the network data are not dependent on any specific technology to carry out the task. Protocols describe what must be done to communicate, not how the task is to be completed.This is the reason that enables different kind of devices such as telephones and computers to use the same network infrasturcture to communicate.


Networking professionals use two networking models to comminicate within the industry, they are protocol models and reference models. Both were created in the 1970’s.

A protocol model is a model that closely matches the structure of a particular protocol suite. The hierarhical set of related protocols in a suite typically represents all the functionality required to interface the human network with the data network. The TCP/IP model is a protocol model because it describes the functions that occur at each layer of protocols with in the protocol suite.

A refrence model provides a common referecen for maintaining the consistency within alkl types ofn etwork protocols and services. The primary function of a refercen model is to aid in clearer understanding of the functions and process involved. The OPEN SYSTEMS INERCONNECTION (OSI) the most well known reference model.


In 1978, the International Organization for Standardization (ISO) released a set of specifications that described network architecture for connecting dissimilar devices. The original document applied to systems that were open to each other because they could all use the same protocols and standards to exchange information.


The topmost layer of the OSI reference model, is the application layer. This layer relates to the services that directly support user applications, such as software for file transfers, database access, and e-mail. In other words, it serves as a window through which application processes can access network services. A message to be sent across the network enters the OSI reference model at this point and exits the OSI reference model’s application layer on the receiving computer.


The presentation layer, defines the format used to exchange data among networked computers. Think of it as the network’s translator. When computers from dissimilar systems need to communicate, a certain amount of translation and byte reordering must be done. Within the sending computer, the presentation layer translates data from the format sent down from the application layer into a commonly recognized, intermediary format. At the receiving computer, this layer translates the intermediary format into a format that can be useful to that computer’s application layer. The presentation layer is responsible for converting protocols, translating the data, encrypting the data, changing or converting the character set, and expanding graphics commands. The presentation layer also manages data compression to reduce the number of bits that need to be transmitted.


The session layer, allows two applications on different computers to open, use, and close a connection called a session. (A session is a highly structured dialog between two workstations.) The session layer is responsible for managing this dialog. It performs name-recognition and other functions, such as security, that are needed to allow two applications to communicate over the network.


The transport layer, provides an additional connection level beneath the session layer. The transport layer ensures that packets are delivered error free, in sequence, and without losses or duplications. At the sending computer, this layer repackages messages, dividing long messages into several packets and collecting small packets together in one package. This process ensures that packets are transmitted efficiently over the network. At the receiving computer, the transport layer opens the packets, reassembles the original messages, and, typically, sends an acknowledgment that the message was received. If a duplicate packet arrives, this layer will recognize the duplicate and discard it.


The network layer, is responsible for addressing messages and translating logical addresses and names into physical addresses. This layer also determines the route from the source to the destination computer. It determines which path the data should take based on network conditions, priority of service, and other factors. It also manages traffic problems on the network, such as switching and routing of packets and controlling the congestion of data.


The data-link layer, sends data frames from the network layer to the physical layer. It controls the electrical impulses that enter and leave the network cable. On the receiving end, the data-link layer packages raw bits from the physical layer into data frames. The electrical representation of the data is known to this layer only.


The bottom layer of the OSI reference model, is the physical layer. This layer transmits the unstructured, raw bit stream over a physical medium (such as the network cable). The physical layer is totally hardware-oriented and deals with all aspects of establishing and maintaining a physical link between communicating computers. The physical layer also carries the signals that transmit data generated by each of the higher layers.


The TCP/IP protocol does not exactly match the OSI reference model. Instead of seven layers, it uses only four. Commonly referred to as the Internet Protocol Suite, TCP/IP is broken into the following four layers:


Network access layer communicates directly with the network. It provides the interface between the network architecture (such as token ring, Ethernet) and the Internet layer.


The Internet layer, corresponding to the network layer of the OSI reference model, uses several protocols for routing and delivering packets. Router are protocol dependent, they function at this layer of the model and are used to forward packets from one network or segment to another. Several protocols work within the Internet layer.


The transport layer, corresponding to the transport layer of the OSI reference model, is responsible for establishing and maintaining end-to-end communication between two hosts. The transport layer provides acknowledgment of receipt, flow control, and sequencing of packets. It also handles retransmissions of packets. The transport layer can use either TCP or User Datagram Protocol (UDP) protocols depending on the requirements of the transmission.


Corresponding to the session, presentation, and application layers of the OSI reference model, the application layer connects applications to the network. It contains all the higher-level protocols.


The OSI and TCP/IP reference models have much in common. Both are based on the concept of a stack of independent protocols. Also, the functionality of the layers is roughly similar. For example, in both models the layers up through and including the transport layer are there to provide an end-to-end, network-independent transport service to processes wishing to communicate. These layers form the transport provider. Again in both models, the layers above transport are application-oriented users of the transport service.

The differece between OSI and TCP/IP model is that the Application layer of TCP/IP model operates at the upper three layers of OSI model, they are application layer, presentation layer and session layer, also the Network layer of TCP/IP model works at the lower two layers of OSI model that are, data link layer and physical layer.


The TCP/IP suite of protocols is the set of protocols used to communicate across the internet. It is also widely used on many organizational networks due to its flexibility and wide array of functionality provided. Microsoft who had originally developed their own set of protocols now is more widely using TCP/IP, at first for transport and now to support other services.



Internet Protocol (IP) is a packet-switched protocol that performs addressing and route selection. As a packet is transmitted, this protocol appends a header to the packet so that it can be routed through the network using dynamic routing tables. IP is a connectionless protocol and sends packets without expecting the receiving host to acknowledge receipt. In addition, IP is responsible for packet assembly and disassembly as required by the physical and data-link layers of the OSI reference model. Each IP packet is made up of a source and a destination address, protocol identifier, checksum (a calculated value), and a TTL (which stands for “time to live”). The TTL tells each router on the network between the source and the destination how long the packet has to remain on the network. It works like a countdown counter or clock. As the packet passes through the router, the router deducts the larger of one unit (one second) or the time that the packet was queued for delivery. For example, if a packet has a TTL of 128, it can stay on the network for 128 seconds or 128 hops (each stop, or router, along the way), or any combination of the two. The purpose of the TTL is to prevent lost or damaged data packets (such as missing e-mail messages) from endlessly wandering the network. When the TTL counts down to zero, the packet is eliminated from the network.


The key fields of the ipv4 are as follows:-

      • SOURCE ADDRESS: Sender’s ip address
      • DESTINATION ADDRESS : Receiver’s ip address
      • TIME TO LIVE (TTL): Numeber of hops a packet must traverse before getting discarded.
      • TYPE OF SERVICE (TOS): It is for a sending host to specify a preference for how the datagram would be handled as it makes its way through an internet.
      • PROTOCOL: This field defines the protocol used in the data portion of the IP datagram.
      • FLAG AND FRAGMENT: A three-bit field follows and is used to control or identify fragments
      • VERSION: Protocol version.
      • INTERNET HEADER LENGTH: The second field (4 bits) is the Internet Header Length (IHL) telling the number of 32-bitwordsin the header.
      • PACKET LENGTH: This 16-bit field defines the entire datagram size, including header and data, in bytes.

Before an IP packet can be forwarded to another host, the hardware address of the receiving machine must be known. The ARP determines hardware addresses (MAC addresses) that correspond to an IP address. If ARP does not contain the address in its own cache, it broadcasts a request for the address. All hosts on the network process the request and, if they contain a map to that address, pass the address back to the requestor. The packet is then sent on its way, and the new information address is stored in the router’s cache.


Some important fields of ARP’s header are as follows:-

      • HARDWARE TYPE: This field specifies the Link Layer protocol type
      • PROTOCOL TYPE: This field specifies the upper layer protocol for which the ARP request is intended
      • HARDWARE LENGTH: Length of a hardware address
      • PROTOCOL LENGTH: Length (in octets) of alogical addressof the specified protocol
      • OPERATION: Specifies the operation that the sender is performing
      • SENDER HARDWARE ADDRESS: Hardware (MAC) address of the sender.
      • SENDER PROTOCOL ADDRESS: Upper layer protocol address of the sender.
      • TARGET PROTOCOL ADDRESS: Hardware address of the intended receiver.
      • TARGET HARDWARE ADDRESS: Upper layer protocol address of the intended receiver.

The TCP is responsible for the reliable transmission of data from one node to another. It is a connection-based protocol and establishes a connection (also known as a session, virtual circuit, or link), between two machines before any data is transferred. To establish a reliable connection, TCP uses what is known as a “three-way handshake.” This establishes the port number and beginning sequence numbers from both sides of the transmission.


Following are some important fields of TCP header:

      • SOURCE PORT: Identifies the sending port.
      • DESTINATION PORT: Identifies the receiving port.
      • SEQUENCE NUMBER: This is the initial sequence number.
      • ACKNOWLEDGEMENT NUMBER: A 32 bit acknowledgement number.
      • DATA OFFSET: Specifies the size of the TCP header in 32-bit words.

A connectionless protocol, the UDP, is responsible for end-to-end transmission of data. Unlike TCP, however, UDP does not establish a connection. It attempts to send the data and to verify that the destination host actually receives the data. UDP is best used to send small amounts of data for which guaranteed delivery is not required. While UDP uses ports, they are different from TCP ports; therefore, they can use the same numbers without interference.


Some key headers of UDP are as follows:

    • SOURCE PORT: This field identifies the sending port.
    • DESITNATION PORT: This field indentifies the receiving port
    • LENGTH: A 16-bit field that specifies the length in bytes of the entire datagram
    • CHECKSUM: The 16-bitchecksumfield is used for error-checking of the headeranddata.


There are millions of computers in use on the web and billions of messages traversing networks at any given time, so prper addresing is essential to make sure that the sent messages arrives intact at the proper destination. Addressing of data happens in three different layers of the OSI model. The PDU at each layer adds address information for use by the peer layer at the destination.

CHAPTER 2: ROUTING Fundamentals


Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network, electronic data networks such as the Internet, and transportation networks. Our main concern will be routing in packet switched networks. In packet switching networks, routing directs packet forwarding, the transit of logically addressed packets from their source toward their ultimate destination through intermediate nodes; typically hardware devices called routers, bridges, gateways, firewalls, or switches. General-purpose computers with multiple network cards can also forward packets and perform routing, though they are not specialized hardware and may suffer from limited performance. The routing process usually directs forwarding on the basis of routing tables which maintain a record of the routes to various network destinations. Thus, constructing routing tables, which are held in the routers’ memory, is very important for efficient routing. Most routing algorithms use only one network path at a time, but multipath routing techniques enable the use of multiple alternative paths.



Static routing is manually adding routes to the routing table, routes through a data network are described by fixed paths (statically). These routes are usually entered into the router by the system administrator. An entire network can be configured using static routes, but this type of configuration is not fault tolerant. When there is a change in the network or a failure occurs between two statically defined nodes, traffic will not be rerouted. This means that anything that wishes to take an affected path will either have to wait for the failure to be repaired or the static route to be updated by the administrator before restarting its journey. Most requests will time out (ultimately failing) before these repairs can be made. There are, however, times when static routes make sense and can even improve the performance of a network. Some of these include stub networks and default routes.


Dynamic routing performs the same function as static routing except it is more robust. Static routing allows routing tables in specific routers to be set up in a static manner so network routes for packets are set. If a router on the route goes down the destination may become unreachable. Dynamic routing allows routing tables in routers to change as the possible routes change. Dynamic routing uses routing protocols for routing information automatically over the internertwork.


Before going further we need to examine the difference between static and dynamic routing.


Before going in to the details of dynamic routing we must understand what are routing protocols. Routing protocols implement algorithms that tell routers the best paths through internetworks. Routing protocols provide the layer 3 network state update. In short, routing protocols route datagrams through a network. Routing is a layer 3 function, thus, routing and routed protocols are network-layer entities. Routing tables on the layer 3 router are populated by information from routing protocols. A routed protocol will enter an interface on a router, be placed in a memory buffer, then it will be forwarded out to an interface based on information in the routing table


Dynamic routing protocols can be divided in to the following broad catagories.



Classful routing protocols do not send subnet mask information in routing updates. This was at the time when network address were allocated on the basis of classes i.e A, B or C. These routing prtocols did not include subnet mask in routing update because the the network mask was determined by first octet of the network address.

Classfull routing protocols can still be used in today’s networks but they cannot be used in all situations because they do not include the subnet mask. Classfull routing protocols cannot be used where the network is subnetted using more then one subnet mask, in other words we can say that classfull routing protocols do not support variable-lenght subnet mask (VLSM). In the following figure the classfull version of the network support similar subnet masks i.e all /24.


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