A field in a datagram, or IP packet header. Analagous to the type field in a network frame. The value it contains specifies which high-level protocol was used to create the message carried in the DATA area of the datagram. It specifies the format of the data area. The mapping between the protocol and the integer value used in this field must be administered by a central authority to guarantee agreement.

Some protocol numbers:

1 - ICMP
6 - TCP
17 - UDP
88 - IGRP
89 - OSPF