By default in Ubuntu (and probably other distributions), when you log into the SSH server, it will do a reverse DNS lookup of the client for security reasons.
That’s fine, but it’s kind of annoying when you don’t have a working reverse DNS for your IP address. Like for all my internal addresses.
To turn it off (thanks to http://ubuntuforums.org/showthread.php?t=577616):
Edit /etc/ssh/sshd_config and add the following line: