Glossary · Networking

ICMP (Internet Control Message Protocol)

ICMP is the IP companion protocol that carries error reports and diagnostic messages, such as echo requests used by ping and unreachable notices.

Updated October 6, 2026

ICMP (Internet Control Message Protocol) is a network-layer protocol used by routers and hosts to report errors and answer diagnostic queries. It has no ports and does not carry application data.

Why it matters

Ping uses ICMP echo request and echo reply. Traceroute relies on ICMP "time exceeded" replies from each router. "Destination unreachable" and "fragmentation needed" messages let senders adapt, and path MTU discovery depends on them. Blocking all ICMP can therefore cause strange stalls, even though a blocked ping is often harmless.

Example

$ ping -c 1 203.0.113.10
64 bytes from 203.0.113.10: icmp_seq=1 ttl=57 time=12.4 ms

Type 8 (echo request) went out and type 0 (echo reply) came back in 12.4 ms.

Common mistakes

  • Dropping every ICMP type in the firewall and breaking path MTU discovery.
  • Concluding a server is down because it does not answer ping. Many hosts deliberately ignore it.
  • Forgetting that ICMPv6 is essential for IPv6 operation and must not be blocked wholesale.

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