In the IP header, the protocol type field comprises 8 bits and is used to demultiplex incoming datagrams. The protocol type defines the subsequent protocol of the next higher layer, i.e. the transport protocol. The most important protocol types are the ICMP protocol( encoding 1), IGMP protocol (2), TCP protocol (6), EGP protocol (8) and the UDP protocol (17).
The encodings for the higher protocols are published in the Assigned Numbers.