There is one exception to this rule. Pings are allowed through. A ping is a simple tool to check network connectivity.

Remember those old submarine movies where the scene shows the entire control room dead silent while the sonar operator is concentrating hard as he can listening for a return "ping" to the sound sent out from the sub? When he finally gets the return "ping", he can accurately guage the distance from the sub to that destroyer headed their way at top speed.

Pings play an equally, if not so life-preserving, role in our computer networks. When one computer pings another, it sends a TCP/IP packet across the internet instead of a sound burst. The target computer responds to the "ping" with a return packet called a "pong".

The originating computer then displays the time in milliseconds for the packet to get to the target and return. This process is repeated for a set number of packets or until the ping process is stopped.

On a unix-like system like OpenBSD, ping's output looks like this:

PING ( 56 data bytes
     64 bytes from icmp_seq=0 ttl=255 time=2.042 ms
     64 bytes from icmp_seq=1 ttl=255 time=1.160 ms
     64 bytes from icmp_seq=2 ttl=255 time=1.122 ms
     64 bytes from icmp_seq=3 ttl=255 time=1.094 ms
     64 bytes from icmp_seq=4 ttl=255 time=1.128 ms
--- ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max/std-dev = 1.094/1.309/2.042/0.367 ms

This definition opened in a different browser window, so you can close this window to return to the original window.