The IP resides within layer 3 (network layer) of the OSI Model. It provides end-to-end transport of data units through internets using connectionless services. Being connectionless, IP does not provide reliable data transfer, but this is not an issue if the upper layers provide reliability and error control.

An IP host must encapsulate data into IP headers, which are then passed to the data link (such as Ethernet). The protocol at the data link layer then encapsulates the IP header with the data into its own data unit (the datagram). The datagram is then passed down to the physical layer, where it is passed over the network as a serial bit stream (with possible encapsulation again, depending on the technology used).

For data to leave the local network, it must be sent to a router. Routers are network layer devices and are capable of processing the Ethernet and the IP headers. If the data is to be passed to another network, the Ethernet (or data link header) is stripped from the data, and the IP header is then processed.

Before transmitting the data over a port to the next network, the router must create a new IP header and place the data (consisting of the TCP header, possibly application header, and user data) into the IP header. The datagram is then given to the data link layer (which may now be X.25 ISDN, Frame Relay, or even Switched 56), and the whole process is repeated.

IP does have its limitations, the biggest being the number of addresses available. As you will see when we discuss IP addresses, there is a severe limitation in the number of addresses that IP can support. This issue has brought about the need for a replacement to IP. Internet Protocol next generation (IPng) provides a 16-byte address rather than a 4-byte one.

The primary function of IP is to provide routing information for data being transported through internets. Any error control is provided by the Internet Control Message Protocol (ICMP), which resides at layer 3 as well. This protocol does not provide error control but merely reports errors to the originating hosts.

IP is not a requirement for TCP. The TCP protocol can use almost any network layer protocol for delivery as long as the protocol is capable of providing routing services and supports the interfaces between the two layers. Remember that the concept of layering was to allow various layers of a protocol to be changed without affecting the layers above or below it.

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Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 149-155.