The IP protocol and its associated routing protocols are possibly the most significant of the entire TCP/IP suite. IP is responsible for:
IP addressing - The IP addressing conventions are part of the IP protocol. (Chapter 3, Planning Your Network, describes IP addressing in complete detail.)
Host-to-host communications - IP determines the path a packet must take, based on the receiving host's IP address.
Packet formatting - IP assembles packets into units known as IP datagrams. Datagrams are fully described in "Internet Layer".