Appendix C: IP Protocol Numbers
In Chapter 2, “Understanding OSI and TCP/IP,” you saw the IP header datagram format. An important ﬁeld for ﬁltering and blocking trafﬁc within the IP header is the protocol ﬁeld. The protocol ﬁeld is 8 bits long. And because the ﬁeld is 8 bits, it can accommodate 256 (or 28) different protocols.