IP address
An IP address serves two principal functions. It identifies the host, or more specifically its network interface, and it provides the location of the host in the network, and thus the capability of establishing a path to that host. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."
The header of each IP packet contains the IP address of the sending host, and that of the destination host.
List of IP protocol numbers
Below are the Internet Protocol numbers found in the "Protocol" field of the IPv4 header and the "Next Header" field of the IPv6 header. The same values are used in both versions of the field and define the layout of the header that will immediately follow the IPv4 or IPv6 header. Both the "Protocol" and "Next Header" fields are eight bits wide. The numbers used in these fields are managed and assigned by the Internet Assigned Numbers Authority (IANA).