Destination and Next Hop Addresses

What is the relationship between the destination address in a datagram header and the address of the next hop to which the datagram is forwarded?The Destination IP address field in a datagram contains the address of the Ultimate destination.When a router receives a datagram,the router extracts the destination address D,and uses it to compute the address of the next router to which datagram should be sent,N.Although the datagram is sent directly to address N,the header in the datagram retains destination address D.All routes are computed using IP address.After computing the address of a next hop,N,IP software uses the address binding to translate N to an equivalent hardware address for transmission.

Physical Network Connection With Routers

The basic hardware component used to connect heterogeneous networks is a Router.Physically,routers resembles bridges that means each router is a special purpose hardware system dedicated to the task of interconnecting networks.Like a bridge,a contains a processor and memory as well as a separate I/O interface for each network to which it connects.The network treats a connection to a router, the same as the connection to any other computer.

The most important that you have to kept in your mind about router is that it is not only used to connect two similar technology,it can also be used to connect two different types of network technology. Router can connect two LAN's,a LAN and a WAN,or two WAN's.Furthermore,when a router connects two network in the same general category,the networks do not need to use the same technology.For ex:- A router can connect an Ethernet LAN to a frame relay network.Thus,each cloud represents an arbitrary network technology.On summarizing this whole,we can say that a router is a special purpose system dedicated to the task of interconnecting networks.A router can interconnect networks that use different technologies,including different media,physical addressing schemes or frame formats.

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Goal of Internet

The main goal of internetworking is to provide a universal service across heterogeneous networks.To provide universal service among all computers on an internet,routers must agree to forward information from a source on one network to the specified destination on the other network.The task is complex because as all we know that different frame formats and different addressing schemes are used by different networks.Hence,to overcome this problem protocol software is needed on computers and routers to make universal service possible.The main task of these internet protocols is to overcome the differences in frame formats and physical addresses to make communication possible among networks that use different technologies.
For knowing more about protocols for internetworking you can visit:
http://networkingconcepts4u.blogspot.com/2009/02/protocol-for-internetworking.html

Internet Architecture

With the help of routers it is possible for an organization to choose network technologies appropriate for each need,and to use routers to connect all networks into a single network.As all we know that router is used exactly two connections but commercial routers can connect more than two networks.Thus,a single router could connect any number of networks.However,an organization seldom uses single router to connect all of its networks.There are two main reasons behind it:

1.As all we know that router must forwarded each packet,the processor in given router is insufficient to handle the traffic passing among an arbitrary number of networks.

2.The second reason behind this is redundancy,because redundancy improves reliability.To avoid a single point of failure,protocol software continuously monitors internet connection,and instructs the routers to send traffic long alternative paths when a network or router fails.

Thus,when planning an internet,an organization must choose a design that meet's the organization's need for reliability,capacity and cost.In particular,the exact details of internet topology often depends on the bandwidth of the physical networks,the expected traffic,the organization's reliability requirements,and the cost and performance of available router hardware.

Connection between a NIC and a network

The type of connection used between a NIC and a network depends on that network technology.In some technologies,the NIC contains most of the necessary hardware,and attaches directly to the network medium using a single cable or optical fiber.In many other technologies,the NIC does not contain all the electronic circuitry needed to attach directly to the network.Instead,the cable from a NIC attaches to an additional electronic component that then attaches to the network.Surprisingly,the exact details of the connection between a NIC and a network are not determined by the technology that means a given network technology can support multiple wiring schemes.Let take the example of Ethernet technology in which technology i.e Ethernet remains same while the wiring scheme changes.Wiring scheme can be Thick Ethernet Wiring,Thin Ethernet Wiring and Twisted pair Ethernet Wiring.

Thin Ethernet Wiring

A second generation of Ethernet wiring uses a thinner,more flexible cable than the original thick wiring.Formally named as a 10 Base2 and informally named as thin wire Ethernet or Thinnet.This wiring scheme differs from Thicknet wiring scheme in three important ways.First,Thinnet generally costs less to install and operate than Thicknet.Second,because the hardware that performs the transceiver function is built into the NIC,no external transceivers are needed.Third,Thinnet does not use an AUI cable to attach the NIC to a communication medium.Instead,Thinnet attachs directly to the back of each computer using BNC connector.In a Thinnet installation,a coaxial cable stretches between each pair of machines.The cable doesn't need to follow a straight line-it may lie loosely on the tabletop between computers,run under floor,or run in a conduit.Although,wiring for a thin Ethernet appears to be completely different than the wiring for a thick Ethernet,the two schemes share several important properties.Both thick and thin cables are coaxial,meaning that they shield signals from outer interference.Both thick and thin cables requires termination,and both use the bus topology.Most important,because two wiring systems have similar electrical characteristics,that means signals propagate along the cable is in the same way.

Twisted Pair Ethernet

This type of Ethernet wiring is known as Third Generation of Ethernet wiring.This generation is completely differ from its previous two generation that are thick and thin Ethernet because it does not use a coaxial cable.This generation Ethernet typically known as twisted pair Ethernet or TP Ethernet and its first version was formally known as 10BaseT.The wiring for Fast Ethernet is known as 100BaseT,and wiring for Gigabit Ethernet is known as 1000BaseT.Instead of shared medium,the new technology extends the idea used with connection multiplexing in which an electronic device serves as the center of the network.
The basic electronic device is known as an Ethernet Hub.Like the other wiring schemes,twisted pair requires each computer to have a network interface card and a direct connection from NIC to the network.The connection uses unshielded twisted pair wiring with RJ-45 connectors which are the largest versions of the modular connectors used with the telephones.The connector on one end of a twisted pair cable plugs into the network interface on a computer and the connector on the other end plugs into the hub.
Thus,each computer has a dedicated connection to the hub device.Electronic components in hub emulate a physical cable,making the entire system operate like a conventional Ethernet.For ex:A computer attached to a hub must have a physical Ethernet Address that means each computer must use the CSMA/CD to access the network and the standard Ethernet Frame Format.In fact,software does not distinguish between thick Ethernet,thin Ethernet and twisted pair Ethernet because the network interface handles the details and hides any differences.