End systems are connected together by communication links. There are various types of communication links, which are made of numerous types of physical media, comprising twisted pair, fiber optics, radio links and coaxial cable. Different links can transmit data at different rates. The link transmission rate is generally known as bandwidth of the link, which is usually measured in bits/second. The highest the bandwidth, the more is the capacity of channel. End systems aren't usually directly attached to each other through a single communication link. In its place, they are indirectly connected to each other via intermediate switching devices called routers. A router takes a chunk of information which is arriving on one of its incoming communication links and forwards that chunk on one of its outgoing communication links. In the jargon of computer networking, chunk of information is known as a packet. The path that packet takes from sending end system, by a series of communication links and routers, to receiving end system is called a route or path through network. Instead of providing a dedicated path between communicating end systems, Internet uses a technique termed as packet switching which allows multiple communicating end systems to share a path, or parts of a path, at the same time. In the same manner as a router, there is another special machine which is called gateways used in the network which allows different networks to talk to Internet, which uses TCP/IP.