control code for "disconnect" or "hang up
". Some modems are foolish enough to interpret this as the software actually telling it to disconnect, and thus sending this string with a dialup
modem will often result in you being knocked offline
The problem was that the easily amused on IRC would abuse this by going into a populated channel that they thought was particularly lame, e.g. #windows, and typing
/ctcp #windows PING +++ATH0
...which would cause about 1/3 of the channel to spontaneously ping out, since IRC clients, on receiving a "PING" command, are supposed to echo back whatever string was sent with it (usually a timestamp).
People eventually got smart about it, and began hacking their modem scripts to ignore +++ATH0, as well as patching or scripting their IRC client to ignore messages containing this string. However, if you still wish to be nasty and denial of service attack someone with this, you need only get a Unix shell on a non-vulnerable connection and, do
ping -c 5 -p 2B2B2B41544829 <target IP address>
"2B2B2B41544829" being the hex code for the bytes spelling out +++ATH0. I don't recommend doing this. No, really, I don't. Heheh. Yes, I've had a cable modem since 1997 or so, why do you ask?