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PROTOCOLS · DIAGNOSTICS
ICMP Message Analyzer
Look up common ICMPv4 and ICMPv6 message types and codes, then use the result to interpret ping failures and other network-control messages.
Message lookup
Ping & traceroute clues
| Observation | Common ICMP clue |
|---|---|
| Ping succeeds | Echo Reply (type 0 in ICMPv4 / 129 in ICMPv6) |
| Ping times out | No reply may mean filtering, loss, routing failure or a silent destination |
| Destination unreachable | ICMPv4 type 3 / ICMPv6 type 1 |
| Traceroute hop appears | Time Exceeded — ICMPv4 type 11 / ICMPv6 type 3 |
| IPv6 packet too large | ICMPv6 Packet Too Big — type 2 |
A missing ICMP response does not prove that the destination is down. Firewalls, ACLs, rate limiting and host policy can suppress control messages.
Engineering notes
ICMP is a network-layer control and error-reporting protocol carried with IP. It is not a transport protocol and does not make IP reliable. ICMP messages can report delivery and forwarding conditions, support diagnostics such as echo, and provide information needed by mechanisms such as Path MTU Discovery.
Use the analyzer as a reference aid. Exact device behaviour, filtering and generated codes depend on the operating system, firewall and network equipment.