Above IP, different protocols carry data in different ways, and choosing or permitting the right one is a daily networking task. The IP header has a protocol field that tells the receiver what comes next: 6 for TCP, 17 for UDP, 1 for ICMP, 47 for GRE, 50 for ESP and 51 for AH. You rarely need these numbers on the exam, but knowing that ICMP, GRE and IPsec's ESP and AH sit directly on IP, with no port numbers, explains why firewalls and NAT devices must treat them differently from ordinary TCP and UDP traffic.
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