End systems are linked together through communication links. There are several types of communication links that are made of distinct types of physical media, by including fiber optics, coaxial cable, twisted pair, and radio links. Distinct links can transmit data at distinct rates. The link transmission rate is frequently called the bandwidth of the link that is typically measured in bits/second. The highest the bandwidth, the more is the capability of the channel. Usually end systems are not directly attached to each other by a single communication link. Instead, they are indirectly linked to each other through intermediate switching devices termed as routers. A router takes a chunk of information arriving on one of its incoming communication links & forwards that chunk of information on one of its outgoing communication links. In jargon of computer networking, the chunk of information is called as a packet. The path which the packet takes from the sending end system, by series of communication links & routers, to the receiving end system is termed as a route or path through the network. Instead of providing a dedicated path among communicating end systems, the Internet employ a technique known as packet switching which allows multiple communicating end systems to share a path, or parts of a path, at the similar time. Similar to a router, there is another special machine called gateways used in the network that let different networks to talk to the Internet that uses TCP/IP.

Packet switching is utilized to avoid long delays in transmitting data over the network. Packet switching is a technique, which restricts the amount of data which a computer can transfer on each turn. Packet switching lets several communications to proceed simultaneously. Each of the packets has a header which specifies the computer to which the packet must be delivered and the destination is denoted using computer's address. Computers which share access to a network take turns in sending packets. On every turn, a given computer sends one packet. On the Internet IP uses this packet switching concept to deliver messages If the destination address does not exist on the local network, this is the responsibility of that network's router to route the message one step closer to its destination. This procedure continues until the destination machine claims the message packet.