I came across a post on Darrel
Norton's blog about learning to use Ethereal for debugging netowrk
traffic. The tutorial comes from Chris Sanders. Check out part
one here. From there
you can follow the rest of the series. You can download Ethereal here. I have personally
used Packetyzer which
is a Windows UI on top of Ethereal. Best part of both of these are that
they are free.
Free is good!