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ICMP: The Network's Error and Diagnostic Protocol

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ICMP: error reporting and network diagnostics

ICMP (Internet Control Message Protocol) is IP's error and diagnostic companion. It carries messages that IP itself can't — 'your packet couldn't be delivered because...'

Unlike TCP and UDP, ICMP has no port numbers. It uses type and code fields to identify message purpose. Many network problems trace back to ICMP being blocked unnecessarily.

Packets in motion — click to inspect:

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