Hostname from Internet address?

[Narayan S. Raja]

Given a hostname (e.g. zip.foo.bar.edu), one
can use a network information program like
nslookup to find its Internet address.  How
about the other way round?  Given an internet
address (e.g. 1.2.3.4), how would one find the 
complete hostname?

(Assuming, of course, that it's not available
in /etc/hosts or equivalent host database).


Narayan Sriranga Raja.

PS:  One could "mail unlikely-username@\[1.2.3.4\]"
       and wait for the mail to bounce back
       from the MAILER-DAEMON (usually giving
       the hostname), but surely there must be
       a simpler way.