ICMP Explained: How It Works and What to Look For
ICMP carries IPv4 diagnostic and error messages.
Published by Protocol Explorer · Reviewed
How ICMP works
A host can send an Echo Request and receive an Echo Reply. Routers and hosts may also report conditions such as unreachable destinations or expired TTL.
What to look for in an exchange
The type and code identify the message. Errors often include part of the packet that caused them, helping the sender associate a report with its traffic.
Follow an example step by step
- Read the ICMP type and code to classify the message.
- For Echo messages, match the identifier and sequence between request and reply.
- For an error, inspect the included original-packet information to determine which flow triggered it.
Common interpretation mistake
A missing ping reply can result from filtering or rate limiting and does not prove every application path is down.
Key point to remember
ICMPv6 has related diagnostic functions but uses a separate specification and additional message types.
Explore the protocol
The ICMP overview is free without an account. Its full interactive lesson is included in Protocol Explorer Pro, a one-time $4.99 purchase; an account is required to buy Pro and restore the licence across devices.
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Specification
Read the primary specification for ICMP for the authoritative protocol details. The interactive scenario is a simplified teaching model, not a capture of your network.
Continue learning
How to read a packet · TCP · UDP · HTTPS