UNIT 2: Internetworking Concept, Devices, Internet basics, History & amp, Architecture

Brief history and growth of Internet:

  1. A network is a group of connected communication devices such as computers and printers. An internet (note the lowercase letter i) is two or more networks that can communicate with each other.
  2.  The most notable internet is called the Internet (uppercase letter I). A collaboration of more than hundreds of thousands of interconnected networks.
  3.  In mid-1960s, mainframe computers in research organizations were stand alone devices. Computers from different manufactures were unable to communicate with one another. The Advanced Research Project Agency (ARPA) in the department of defense (DoD) was interested in finding a way to connect computers so that the researchers they funded could share their findings, thereby reducing costs and eliminating duplication of effort.
  4.  In 1967, at an Association for computing Machinery (ACM) meeting, ARPA presented its ideas do ARPANET, a small network of connected computers. The idea was that each host computer (not necessarily from the same manufacture) would be attached to a specialized computer, called an interface message processor (IMP). The IMPs, in turn, would be connected to one another. Each IMP had to be able to communicate with other IMPs as well as with its own attached host.
  5.  By 1969, ARPANET was reality. Four nodes, at the University of California at Los Angeles (UCLA), the University of California at Santa Barbara (UCSB, Stanford Research Institute (SRI), and the University of Utah, were connected via the IMPs to form a network. Software called the network control protocol (NCP) provided communication between the hosts.
  6.  In 1972, Vint Cerf and BobKahn, both of whom were part of the core ARPANET group, collaborated on what they called the internetting project.
  7.  Cerf and Kahn’s landmark 1973 paper outlined the protocols to achieve end-to-end delivery of packets. This paper on Transmission Control Protocol (TCP) included concepts such as encapsulation, the datagram, and the functions of a gateway. Shortly thereafter, authorities made a decision to split TCP into two protocols: Transmission Control Protocol (TCP) and Internetworking Protocol (IP). IP would handle datagram routing while TCP would be responsible for higher-level functions such as segmentation, reassembly, and error detection. The internetworking protocol became known as TCP/IP.
  8.  Today most end users who want internet connection use the services of Internet service Providers (ISPs). There are international service providers, national service provides, regional service providers, and local service providers. The internet today is run by private companies, not the government. Shows a conceptual (not geographic) view of the internet.



Need for internet:

  1. Resource sharing: goal is to make all program equipments and especially data available to anyone on the network without regard of physical locator.
  2. High reliability: This can be provided by alternate source of supply. Presence of multiple CPU‟s means that if one goes down, the other may be able to take over its work at a reduced performance.
  3. Saving money: Data are stored in one or more shared file server machine in case of internet. Here the users are called client and whole arrangement in client server model so that anybody can uses in the internet.
  4. Communication medium : B using internetworking, it is easy for two or more people who live apart can get in touch and can share their data. So it is a powerful communication medium.

Problem of internet working:

  1. Addressing: Each net may have its own addressing approach. Ethernet uses 6-byte ID, Telephone uses 10-digit numbers.
  2. Bandwidth and Latency: Link bandwidth would vary from net to net from bits/sec to Gb/sec. Latencies can be from μs to several secs.
  3. Packet size: In general varies between networks
  4. Loss rates & QoS: All networks vary considerably in these. These depend on traffic volume, congestion, service demands
  5. Packet routing: Routing of packets may be handled differently by different nets.

Incompatibility issues of internet working

Hardware Issue:

  1. Some additional hardware is used to connect physically distinct computer networks. This hardware component is most commonly a router. A router is a special-purpose computer that is used specifically for internet working purposes.
  2. A router has a processor (CPU) & memory like any other computer. However, it has more than one I/O interface that allows it to connect to multiple computer networks. A network connects to a router in the same way as it connects to any other computer. A router connects two or more computer networks. The router is a special computer that has two networks which connect to these two networks. These two networks correspond to the two physical addresses that the router has. A router can connect incompatible networks. That is, networks A &


Fig: A router connects two or more computer networks together

Software Issue:

  1. At the software level, routers must agree about the way in which information from the source computer on one network would be transmitted to the destination computer on a different network.
  2. Since this information is likely to travel via one or more router, there must be a pre specified standard to which all routers must confirm. Packet formats and addressing mechanisms used by the underlying networks may not be the same. Does the router actually perform the conversion and reconversion of the packets corresponding to the different networks formats? Though not impossible, this approach is very difficult and cumbersome.
  3.  Therefore, some networking protocols are requires that can standardize common between incompatible networks. Only then can the concept of universal service be truly realized. In the case of all Internet communication, the TCP/IP suite of protocols makes this possible.
  4. The basic idea is that TCP/IP defines a packet size, routing algorithms, error control method etc universally. TCP/IP solves the problem of connecting heterogeneous networks seamlessly.

Network Control Devices or List different network connecting devices:

  1. Repeater
  2. Hub
  3. Bridge
  4. Switch
  5. Router
  6. Gateway
  7. Modem
  8. Firewall


A repeater is an electronic device that receives a signal and retransmits it at a higher level or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.

Functions of repeater:

  1. Repeater is an electronic device that operates on physical layer of OSI model.
  2. A repeater is used to regenerate the signal.
  3. A repeater allows us to extend the physical length of a network.
  4. A repeater is used to boost the weak signal when the signal loses the strength as it passes along the cable.
  5. A repeater does not amplify the signal.

Uses : A telephone repeater is an amplifier in a telephone line, An optical repeater is an optoelectronic circuit that amplifies the light beam in a optical fiber cable; A radio repeater is a radio receiver and transmitter that retransmit a radio signal.

Advantages of Repeater:

  1. A repeater is used to regenerate the signal.
  2. It can be used to connect two segments.
  3. A repeater allows extending the physical length of a network.
  4. A repeater is used to boost the weak signal when the signal loses the strength as it passes along the cable.
  5. A repeater does not have filtering capacity; It forwards every frame.
  6. Repeaters are cheaper when compared to other networking devices.

Disadvantages of Repeater:

  1. Repeater regenerate the signal doesn‟t amplify the signal
  2. Repeater is not an intelligent device and it works in physical layer
  3. Position of repeater in data communication is important; need to place before signal gets weak.
  4. Traffic cannot be filtered by repeater to ease congestion.


HUB or Concentrator:

A Hub or Concentrator is a device used to connect all of the computers on a star network. It works on the physical layer of OSI reference model. A hub is multiport repeater. When a signal is received on one port of the hub it is generated out all the other ports. This type of device simply passes on all the information it receives, so that all devices connected to its ports receive that information.

Installing Hub is very simple to connecting it to power source & plugging in cables connected to the network interface adaptor in your computer.


As shown in Figure; the hub is a hardware device that contains multiple; independent ports that match the cable type of the network. Most common hubs interconnect Category twisted-pair cable with RJ-45 ends, although Coax BNC and Fiber Optic BNC hubs also exist. The hub is considered the least common denominator in device concentrators. Hubs offer an inexpensive option for transporting data between devices.

There are three different types of HUB:

  1. Passive Hub does nothing with the signal.
  2. An active Hub strengthens and regenerates the incoming signals before sending the data on to its destination.
  3. Intelligent Hub performs some network management & intelligent path selection. It is also used to create multiple level of hierarchy & also perform some network management.

Advantages of HUB:  

  1. No need of configuration.
  2. Active Hub can extend maximum network media distance.
  3. No processing is done at the hub to slow down the performance.

Disadvantages of Hub:

  1. Hub has no intelligence to filter traffic, so all data is sent out to all ports whether it is needed or not.
  2. Passive Hub can greatly limit maximum media distance.
  3. Hub can act as a repeater; networks using them must follow the same rule as repeaters.


A bridge is a device used to connect LAN segments together (i.e. it connects two different segment of same LAN). It operates at physical as well as data link layer. A bridge device filters data traffic or packets at the data link layer meaning that it only passes the packets that are destinated for the other side of network.

Bridges reduce the amount of traffic on a local area network (LAN) by dividing it into two segments. Bridges inspect incoming traffic and decide whether to forward or discard it, this decision is taking by bridge on the basis of bridging table maintained by bridge. This table contains the MAC address of each node present in the LAN. So before sending information the bridge checks the destination address in the table if found it send the frame to the particular node else drop the frame.

They can also be used to divide large networks into smaller segments. Bridges have all the features of repeaters, but can have more nodes, and since the network is divided, there is fewer computers competing for resources on each segment thus improving network performance. Bridges can also connect networks that run at different speeds, different topologies, or different protocols. But they cannot, join an Ethernet segment with a Token Ring segment, because these use different networking standards.

Bridges read the MAC header of each frame to determine on which side of the bridge the destination device is located, the bridge then repeats the transmission to the segment where the device is located. A Bridge examines the each packet as it enters through one of the ports. It first looks at the MAC address of the sender & creates a mapping between the port the senders MAC address.

Functions of bridges:

  1. Bridge divides large network into smaller segments.
  1. Bridge is a connecting device which works on data link layer of OSI model.
  1. Each incoming packet will pass to MAC layer of bridge
  2. Here Physical address of destination is checked if it is destined to another subnet then it is forwarded to that subnet but if that subnet is of different type then LLC will remove header and apply new header.
  3. To identify destination present on which subnet, bridge maintains forwarding table containing information about on which port data has to be handed over for which destination
  4. By analyzing source address of incoming Packet, Bridge upgrades its forwarding table by learning about which device present on which subnet. Bridge does not change physical address.
  5. If bridge is having different LAN on segment then bridge will have separate MAC& LLC layer per type of LAN.


Fig a. Bridge

Types of Bridge:

  1. Transparent Bridges
  2. Source Routing Bridges
  3. Two-Layer Switches
  4. Simple Bridges
  5. Multiport Bridge

Transparent Bridges:

A transparent bridge is a bridge in which the stations are completely unaware of the bridge’s existence. If a bridge is added or deleted from the system, reconfiguration of the stations is unnecessary. According to the IEEE 802.1 d specification, a system equipped with transparent bridges must meet three criteria:

Forwarding: Frames must be forwarded from one station to another.

Learning: The forwarding table is automatically made by learning frame movements in the network.

Loops Problems: Loops in system must be prevented.

Source-route Bridge:

Source-route Bridges were designed by IBM for use on Token ring networks. The SR Bridge derives the entire route of the frame embedded within the frame. This allows the Bridge to make specific decision about how the frame should be forwarded through the network.

Two-Layer Switches:

A two-layer switch is a bridge, a bridge with many ports and a design that allows better (faster) performance. A bridge with a few ports can connect a few LANs together. A bridge with many ports may be able to allocate a unique port to each station, with each station on its own independent entity. This means no competing traffic (no collision, as we saw in Ethernet).

A two-layer switch, as a bridge does, makes a filtering decision based on the MAC address of the frame it received. However, a two-layer switch can be more sophisticated. It can have a buffer to hold the frames for processing. It can have a switching factor that forwards the frames faster. Some new two-layer switches, called cut-through switches, have been designed to forward the frame as soon as they check the MAC addresses in the header of the frame

Advantages of Bridge:

  1. Bridges are simple & significant.
  2. They prevent unnecessary traffic from crossing onto other network segments.
  3. Bridge can reduce the amount of network traffic on segment.
  4. They can connect different network architectures like Ethernet & token ring.

Disadvantages of Bridge:

  1. Bridges are slower than repeater & hubs because they examine each frame source & destination addresses.
  2. A Bridge cannot make decision about routes through the network.


Switch is a small device that joins multiple computers together. It works on physical as well as data link layer. Switch looks nearly identical to hub. Unlike hubs switches are more capable of inspecting the data packets as they are received, determining the source & destination address of that packet & forwarding that packet properly.

A network switch (also called switching hub, bridging hub, officially MAC bridge) is a computer networking device that connects devices together on a computer network, by using packet switching to receive, process and forward data to the destination device. Unlike less advanced network hubs, a network switch forwards data only to one or multiple devices that need to receive it, rather than broadcasting the same data out of each of its ports

A network switch is a multiport network bridge that uses hardware addresses to process and forward data at the data link layer (layer 2) of the OSI model. Switches can also process data at the network layer (layer 3) by additionally incorporating routing functionality that most commonly uses IP addresses to perform packet forwarding; such switches are commonly known as layer-3 switches or multilayer switches.

Types of Switch:

  1. Cut-through Switch: This switch forwards the packets as soon as it is received. No error checking is performed on the packet, so packet is moved through very quickly. This type of switch is relatively inexpensive & minimizes the delay incurred during the processing of packets by the switch.
  2. Store & forward: In this switch, the switch waits to receive the entire packets before beginning to forward it. It performs basic error checking. This type of switch is a shared memory switch, which has a common memory buffer that stores the incoming data from all the ports.
  3. Fragment Free: Fragment free switch works by reading only the port of the packet that enables it to identify the fragments of a transmission.


Router:  Routers work at the network layer of the OSI reference model. The router uses network addresses to determine how to forward a packet. Routers are in increasingly common sight in any network environment. A router is a networking device that forwards data packets between computer networks. Routers perform the “traffic directing” functions on the Internet. A data packet is typically forwarded from one router to another through the networks that constitute the internetwork until it reaches its destination node.

Routers make interconnecting possible. Routers are network device that route the data around the network. Router first examine the data as it receives, then determine the destination address for the data. By using the routing table of defined routes, the router determines the best way for the data to route the destination. Unlike bridges & switches, it uses the hardware configured MAC address to determine the destination of the data; routers use the software configured network address to make decisions. A router connects two or more logical separate networks. It can connect both similar & dissimilar networks. They are often used for WAN.




Types of Routers:

1) Static Routers: Static routing is a form of routing that occurs when a router uses a manually-configured routing entry, rather than information from a dynamic routing traffic. In many cases, static routes are manually configured by a network administrator by adding in entries into a routing table, though this may not always be the case. These Routers are generally used only for small environment.

Main disadvantages of Static router:

  1. Manually entering routes is the time consuming & susceptible to human error or routes are fixed so no flexibility to forward the packets.
  2. If the topology of the network changed, the routers must be manually configured. i.e. Unlikedynamic routing, static routes are fixed and do not change if the network is changed or reconfigured.

2) Dynamic Router: Dynamic router do an automatic discovery of routes or periodically router updates its routing table, which uses external protocols to gather information about the network. Only the first route has to be manually configured. After that additional routes are automatically discovered. The route is decided by the router on the basis of traffic & cost.


  1. Used to connect networks i.e. without router, the internet could not exist.
  2. Connecting two or more independent network.
  3. Used to provide connectivity across wide area network (WAN) links.
  4. It can connect dissimilar LANs & WANs running at different transmission speed & using a variety of protocols.
  5. Diagnose internal or other connectivity problems.


  1. Routers do not have ability to stop broadcast packets from being forwarded to other networks.
  2. Router operates slower than any of the other devices because extra decision making involved.


Fig: Dynamic Routing


A network gateway is an internetworking system capable of joining together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both. Depending on the types of protocols they support, network gateways can operate at any level of the OSI model. Gateway is a application specific interfaces that link all seven layers of OSI model. Gateway is not layer specific it can be support any layer of network.

The term gateway is applied to any device, system or software application that can perform the function of translating data from one format to another. The key feature of the gateway is that it converts the format of data. It is a combination of software & hardware that connects two dissimilar kind of network. It controls the traffic that travel from the inside the network to the internet. The default gateway is the machine IP number that you need to access to get to the rest of the network. It connects two efferent networks like your network to the internet. You will need to know the specific address of your gateway to connect to the internet.


  1. Gateway allows communication between two dissimilar network protocols.
  2. Typically, gateways are used for one of four purposes as e-mail gateway, as an IBM host gateway, as an Internet Gateway, as an LAN gateway.


  1. Gateway becomes bottleneck because the flow of data is slow & the conversion from one data format to another form takes times.

Various Gateway in OSI Reference Model:


  1. Gateways provide full protocol conversion from one proprietary LAN technology to another, i.e. Ethernet to token ring or FDDI or any other standard or protocol rather than encapsulation
  1. It uses higher layers of the OSI model, perhaps through layer 7, the application layer. IBM SNA, Telnet, Internet TCP/IP and other protocols can be converted from network-to-network
  1. Unlike bridges and routers, gateways operate slowly because of protocol conversion. Consequently, they may create bottlenecks of congestion during periods of peak usage.

Architecture of ISP:

A subscriber of an ISP connects to one of the telephone lines of the ISP via modem. ISP is an equipment. The ISP‟s modem routes the subscriber to the remote access server (RAS) to authenticate the subscriber. Once the user is authenticated, the subscriber is as good as connected to the internet. Once he connected to the internet, the user can requests web pages, send/receive emails or files, so on. The user interaction with the internet is coordinated by the RAS (remote Access Server), the ISP allows the user to create an email ID and use it for sending/receiving emails.

The main point is that since the internet users are not always connected to the internet, an ISP stores emails on their behalf on the SMTP server temporarily. Web server performs two operations

  1. It can be used by the ISP to setup a portal
  2. This web can be used to store the web pages created by the subscribers.

These web pages belongs to the ISP‟s portal, can be hosted by different web servers for security and maintenance.

Dial-up network:

Dial-up Internet access is a form of Internet access that uses the facilities of the public switched telephone network (PSTN) to establish a connection to an Internet service provider (ISP) by dialing a telephone number on a conventional telephone line. Dialup internet service is a service that allows connectivity to the internet through a standard telephone line. By connecting the telephone line to the modem in computer and inserting the other end into the phone jack, and configuring the computer to dial a specific number provided by internet service provider (ISP) to access the internet on your computer. Dial up internet service is provided through several ISP. In order to get a dial up internet service a person must definitely have a computer and even more important a modem. There are different types of modems, to the modem. A dial-up remote access connection contains the following components:

  • Remote access client
  • Remote access server WAN infrastructure

Dial-up equipment and the WAN infrastructure


The physical or logical connection between the remote access server and the remote access client is facilitated by dial-up equipment installed at the remote access client, the remote access server, and the WAN infrastructure. The nature of the dial-up equipment and WAN infrastructure varies, depending on the type of connection. The most common methods for dial-up remote access include:

  • Public Switched Telephone Network (PSTN)
  • Integrated Services Digital Network (ISDN)

Specifications of Dial up connection:

  1. Uses the facility of telephone lines(PSTN)
  2. Requires modem of 56Kbps for conversion of digital to analog and vice versa.
  3. Dial-up lines generally support speeds of 2,400 to 9,600 bps.
  4. No dedicated path.

Advantages of DSL:

  1. DSL provides higher-speed access to the Internet.
  2. DSL makes use of existing wire setup for the transmission of data.
  3. It provides higher data rates and a useful facility for a home user.
  4. It has higher download speed than the upstream when compare to ordinary dial up connection.

Virtual Network:

  1. A virtual network is a computer network that consists, at least in part, of virtual network links.
  2. A virtual network link is a link that does not consist of a permanent physical (wired or wireless) connection between two computing devices but is implemented using methods of network virtualization. Only abstract view is available to user .user is not aware or bother about physical structure of network, interconnection, routing decision or presence of networking devices.
  3. It ensures that different computer on different networks can be connected with each other and user can treat them as on single network.
  4. The two most common forms of network virtualization are protocol-based virtual networks a. VLAN: Virtual LAN Virtual LANs (VLANs) are logical local area networks (LANs) based on physical LANs. A VLAN can be created by partitioning a physical LAN into multiple logical LANs using a VLAN ID. Alternatively, several physical LANs can function as a single logical LAN.
  5. The partitioned network can be on a single router, or multiple VLAN’s can be on multiple routers just as multiple physical LAN’s would be. A VLAN can be on a VPN.




Virtual Private Network A virtual private network (VPN) consists of multiple remote end-points joined by some sort of tunnel over another network, usually a third party network. Two such end points constitute a ‘Point to Point Virtual Private Network’. Connecting more than two end points by putting in place a mesh of tunnels creates a ‘Multipoint VPN.00

Cable modem with neat diagram:

  1. Cable modems are one of the ways of accessing the internet.
  2. It provides the internet access to a home user along with cable television access.
  3. As the capacity of the coaxial cable is huge, the unused capacity is utilized to carry internet traffic.
  4. The architecture of internet access via cable modem is shown in fig.
  5. When a user wants to access the internet, he invokes the computer browser.
  6. The browser‟s request reach the Network Interface Card(NIC) of the computer.
  7. A device called splitter is fitted inside the premises of a cable TV user.
  8. The splitter is a device that splits the signals inside a coaxial cable into two parts.
  9. One part of the signal is the television signal and the other part of the signal carries the internet data.
  10. Two separate parts of the signal arrives in separate wires. One wire goes to television set and the other wire carrying the IP packets is connected to the cable modem.
  11. The cable modem converts a computer‟s digital data into analog signals that the wire can carry and vice versa.
  12. The cable company serves each town through a number of central nodes. Each node serves about 500 customers
  13. Many such central nodes are connected via high speed optical fiber links into a single head end .
  14. The head end receives television signals from satellites and internet access via high speed connections with NAPs.

Advantages of cable modem:

  1. A cable modem is used to connect a computer to CCTV
  2. People have cable TV so new wiring is not required.
  3. With a single connection, video and data can be accessed simultaneously.
  4. Transmission speed for downloads is good in cable modems.
  5. Ability to receive live multimedia streams/videos.
  6. Provides more bandwidth.


Leased line with a neat diagram:

A leased line is a dedicated, fixed-bandwidth, symmetric data connection. A permanent telephone connection between two points set up by a telecommunications common carrier. Typically, leased lines are used by businesses to connect geographically distant offices. Unlike normal dial-up connections, a leased line is always active. The fee for the connection is a fixed monthly rate. The primary factors affecting the monthly fee are distance between end points and the speed of the circuit. Because the connection doesn’t carry anybody else’s communications, the carrier can assure a given level of quality.

Telephone companies & ISP‟s have come up with the option of offering more BW from their premises & let the organizations divide it internally the way they want.

UntitledUsed of leased line:

It is used to link two locations together. The first location is typically a corporate office. The second location is typically another corporate office, a data centre that‟s connected to the Internet or a data centre that‟s connected to the company‟s existing Wide Area Network.

Need of leased line:

  1. To provide high speed/ band width dedicated internet line.
  2. It provides bandwidth on demand for a specified duration of time.
  3. More nodes can be added to the network without much modification.

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