Dictionary · Protocols and Ports
ICMP
ICMP — Internet Control Message Protocol
Diagnostics · Beginner
A network-layer protocol for errors and diagnostic messages, including echo requests used by connectivity tests. It does not use port numbers.
Routers and hosts use these messages to say a destination is unreachable, a packet was too big, or a simple echo was answered. Blocking all of it can hide useful errors.
Example: A connectivity test sends an echo request and reports the round-trip time when an echo reply returns.
Common misconception
- These messages do not have TCP or UDP ports. Filtering them is not the same task as closing a port.