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ICMP Message Types

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ICMP: IP's diagnostic and error layer

ICMP (Internet Control Message Protocol) is IP's built-in feedback mechanism. It doesn't carry user data — it carries error reports and diagnostic messages between routers and hosts.

Every IP implementation includes ICMP. Understanding the message types explains how ping, traceroute, PMTUD, and error reporting all work.

Packets in motion — click to inspect:

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