IP connection (firewall)?

[Russell Blau]

I am trying to run a private HTTP server on my Ubuntu machine, for use in
developing and testing a web application.  Because I want it to be private,
I am running it on a random high-numbered port.  My computer is behind a NAT
firewall, so I've set it to forward this one port to my Ubuntu box.

Problem is, I cannot connect to the HTTP server, either from another machine
on my local network or from the Internet.  Even weirder, I *could* connect
to it from my local network at one point, but this no longer works.  (All of
the remote machines that I'm using to try to access the server are running
Windows XP, but I don't think that's the problem, as noted below.)

Here's what I've done to troubleshoot so far --

I can connect to the server using Firefox on my Ubuntu machine
(http://localhost:54321/ works).  However, even from my Ubuntu machine,
http://myipaddress:54321/ doesn't work.

I can connect to other ports on my Ubuntu machine from the Internet, so I
know the port forwarding is set up correctly.  In fact, trying
http://myipaddress:54321/ , which is the port I want to use, returns a
"connection refused" error, but trying http://myipaddress:54322/ , which is
a port that is *not* opened in my NAT firewall, results in "connection timed
out."

"netstat -an" shows that the server program is indeed LISTENING on port
54321.

I also checked iptables, and there is nothing there to block incoming
packets.

What am I missing?  Is there some other firewall program that might have
decided to block these packets?  Is there some low-level way to monitor what
happens to these packets when they arrive at my Ethernet interface?

Thanks in advance for any pointers.