See http://www.wireshark.org/. Wireshark does what tcpdump does, but also displays the captured packets nicely.
For tcpdump see its manual page (if available).
--shmem
_($_=" "x(1<<5)."?\n".q·/)Oo. G°\ /
/\_¯/(q /
---------------------------- \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}