ICMP

Internet Control Message Protocol

In plain language

The behind-the-scenes messages devices use to say a destination is unreachable, and the basis of the ping test.

The metaphor

Like the return-to-sender stamp on a letter: it tells you whether the address was found and how long the trip took.

Going deeper

How it works

A ping sends a small request and waits for a reply.
Traceroute uses the same messages to show each stop along the way.
Firewalls often limit these messages because attackers use them to map a network.
Blocking all of them can break some connections in odd ways.

Network covers the firewall rules that decide whether your office answers pings from the internet.

In practice

IT pings a printer to see whether it is reachable.
A traceroute shows where a slow connection stalls.
A firewall rule allows replies to pings from inside but drops them from outside.

Should my firewall answer pings from the internet?

Usually no. Answering lets outsiders see that your connection is alive, and you lose nothing by staying quiet.

Where WILDCARD handles this

Network

Related terms

Every WILDCARD card exists to solve one named problem. Find yours.