ICMP (Internet Control Message Protocol) is a helper protocol at the Network layer that carries error and diagnostic messages about IP delivery. It does not carry user data and does not use port numbers; instead, each message has a type and a code. Routers and hosts send ICMP messages to report problems such as an unreachable destination, and administrators use ICMP to test reachability. IPv6 has its own version, ICMPv6, which also handles Neighbor Discovery and router advertisements, so it is essential there rather than optional.
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