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ICMP (Internet Control Message Protocol)

ICMP is used for error reporting and diagnostics in IP networks. It communicates network issues such as unreachable hosts, TTL expiry, or congestion.

  • Type examples:

    • Echo Request (8) / Echo Reply (0) – used by ping.
    • Destination Unreachable (3) – host or port unreachable.
    • Time Exceeded (11) – TTL expired (used by traceroute).

Input:

ping -c 4 8.8.8.8

Output:

PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=118 time=12.3 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=118 time=12.1 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=118 time=12.2 ms
64 bytes from 8.8.8.8: icmp_seq=4 ttl=118 time=12.4 ms

--- 8.8.8.8 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3003ms
rtt min/avg/max/mdev = 12.123/12.250/12.400/0.123 ms

Explanation: Verifies IP reachability and measures latency and packet loss.

References: