What does ICMP stand for?

ICMP stands for Internet Control Message Protocol, a network-layer protocol used to report errors and operational information about IP communication.

ICMP was developed for the Internet protocol suite and formally described for IPv4 in RFC 792, published in 1981. Unlike TCP or UDP, it is not primarily a transport protocol for carrying application data. Routers and hosts use ICMP to signal conditions such as unreachable destinations, expired packet lifetimes, or unavailable services.

Two familiar diagnostic tools rely on ICMP. The ping utility sends an Echo Request and looks for an Echo Reply, testing whether a host can respond. Traceroute manipulates the IP time-to-live field and observes ICMP Time Exceeded or Destination Unreachable messages to map a packet's route. ICMP packets do not have TCP- or UDP-style port numbers; IPv4 identifies ICMP with protocol number 1.

A frequent source of confusion is IPv6. IPv6 uses a related but separate protocol called ICMPv6, so the stored expansion remains correct for the standard IPv4 acronym and the general protocol name. ICMP messages are carried inside IP packets and are normally handled as control traffic rather than user data.