Defined in STD 5, RFC 791, is the network layer for the TCP/IP Protocol Suite, a packet-switching protocol that has address and control information so that packets can be routed (see Figure 9-2). Both the Transmission Control Protocol (TCP) and the Internet Protocol (IP) are important. IP provides connectionless, high-level datagram delivery as well as fragmentation and datagram reassembly to support data links having varying maximum-transmission unit (MTU) sizes.

The Internet Protocol itself contains the following information:

The IP addressing setup is critical to the effective routing of IP datagrams through the Internet because every IP address, having specific components and following a given format, can be subdivided and used to generate addresses for sub-networks. Each device on a TCP/IP network is given a unique numerical address (32 bit in IP version 4) that can be divided into two parts: the host number and the network number. The host number identifies a computer on the network and is given by the administrator of the local network, whereas the network number identifies a network and must be given by one of the local Internet Registries (that is, ARIN, RIPE, APNIC, AfriNIC, or LACNIC) if the network is to be connected to the Internet. An Internet Service Provider (ISP) can get blocks of network addresses and thereby assign address space to clients.

See Also: Internet Control Message Protocol (ICMP); TCP/IP or Transmission Control Protocol/Internet Protocol; User Datagram Protocol (UDP).