What is the IP protocol number for ICMP?

What is the IP protocol number for ICMP?

• A. 1
• B. 0
• C. 6
• D. 17

Answer: (A)

In IP, the protocol field in the header tells the network stack which upper-layer protocol should receive the payload. ICMP is assigned the value 1, so any IPv4 packet with this protocol number is meant to be handled by ICMP (for things like echo requests/replies and error messages). By contrast, 6 is used for TCP and 17 for UDP, and 0 is reserved. (Note: in IPv6, ICMP messages use ICMPv6 with a different next‑header value, namely 58.)

In IP, the protocol field in the header tells the network stack which upper-layer protocol should receive the payload. ICMP is assigned the value 1, so any IPv4 packet with this protocol number is meant to be handled by ICMP (for things like echo requests/replies and error messages). By contrast, 6 is used for TCP and 17 for UDP, and 0 is reserved. (Note: in IPv6, ICMP messages use ICMPv6 with a different next‑header value, namely 58.)