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Networking Fundamentals · Foundational

Internet Control Message Protocol (ICMP)

The Layer 3 protocol (IP protocol 1) used for error reporting and diagnostics, such as ping and traceroute.

Definition

ICMP messages are identified by type and code, including Echo Reply (0), Destination Unreachable (3), Redirect (5), Echo Request (8) and Time Exceeded (11). Routers and hosts generate ICMP errors when packets cannot be delivered. ICMP has no ports. Blocking it entirely breaks Path MTU Discovery and troubleshooting.

Example

A router returns ICMP Type 3 Code 1 (host unreachable) when it cannot reach a destination host.

Exam tip

ICMP has types and codes, not ports; it is used by ping (types 8 and 0) and traceroute (type 11 replies).

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