The Internet Protocol (IP) is the central element in the TCP/IP protocol stack. It provides the basic service for all the data traffic in the Internet and other IP-based networks and was specified in RFC 791. The primary task of the Internet Protocol is to hide differences between data transmission layers and to offer a uniform presentation of different network technologies. For example, the Internet protocol can run on top of LAN technologies and SLIP (Serial Line IP) or PPP (Point-to-Point Protocol) over modem or ISDN connections. The uniform presentation of the underlying technology includes an introduction of the uniform addressing scheme (IP address family) and a mechanism to fragment large data packets, so that smaller maximum packet sizes can be transported across networks.
In general, each network technology defines a maximum size for data packets the Maximum Transmission Unit (MTU). The MTU depends on the hardware used and the transmission technology and varies between 276 bytes and 9000 bytes. The Internet layer fragments IP datagrams, which are bigger than the MTU of the network technology used, into smaller packets (fragments). These fragments of a datagram are then put together into the original IP datagram in the destination computer. Section 14.2.3 explains how data packets are fragmented and reassembled.
In summary, the Internet Protocol handles the following functions:
provides an unsecured connectionless datagram service;
defines IP datagrams as basic units for data transmission;
defines the IP addressing scheme;
routes and forwards IP datagrams across interconnected networks;
