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Networking Fundamentals · Intermediate

IP Protocol Number

The value in the IPv4 Protocol field (or IPv6 Next Header) that identifies the encapsulated upper-layer protocol.

Definition

IANA assigns protocol numbers such as 1 ICMP, 2 IGMP, 6 TCP, 17 UDP, 47 GRE, 50 ESP, 51 AH, 58 ICMPv6, 88 EIGRP and 89 OSPF. Protocols identified by protocol number rather than port, such as GRE and ESP, have no ports and must be permitted by protocol in firewalls.

Example

An IPsec ESP packet has IP protocol 50, not a TCP or UDP port.

Exam tip

Do not confuse protocol numbers with port numbers: GRE is protocol 47, not port 47.

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