ICMP: network errors, ping and diagnostics

ICMP carries control and error information for IP networks. Ping uses ICMP Echo, but ICMP also reports conditions such as an unreachable destination, an oversized packet or an expired hop limit.

What ICMP is used for

ICMP carries control and error information for IP. Ping commonly uses Echo messages, while other ICMP messages report conditions such as an unreachable destination or a packet that exceeded its hop limit.

How to read ping results and ICMP errors

Compare the exact ICMP result rather than treating every failed ping as the same problem. A timeout can mean filtering or loss, while Destination Unreachable and Packet Too Big carry specific diagnostic information. A host can also block Echo while its application remains reachable. You can repeat the check from an external point with 2ip ping and traceroute.

When a failed ping does not mean a host is down

Technical reference: RFC 792 and RFC 4443.