What is ICMP?
ICMP (Internet Control Message Protocol) is a Layer 3 protocol used for network diagnostics and error reporting. It lives inside IP packets (protocol number 1) — not TCP or UDP.
The most famous ICMP tool is ping — it tests reachability and measures round-trip time.
ICMP is also used by routers to report errors: *TTL expired*, *Host unreachable*, *Fragmentation needed*, etc.