Debugging a VNC link through routers

[Nigel Gilbert]

I have a Mac at home and my aunt, living 60 miles away, has a Mac in 
her apartment.  Both of us are on broadband, using different ISPs.  I'd 
like to use VNC to help her with problems she has from time to time 
with using her Mac.

She has a ADSL modem/router which I hope I have set up to forward port 
5900 to her Mac.  I have an small local net, with an Apple Airport 
wireless router and cable modem, which I have also set up to forward 
port 5900.  And I have checked that neither Mac has its firewall 
software running.  Her Mac is running a OSXvnc server.

However,  I can't get vnc on my mac to 'see' hers.  My next task is to 
find out where the blockage is.

Is there some software that will identify not only that traffic on port 
5900 is not getting through, but *where* the blockage is?  I have tried 
a traceroute (using mtr), which shows me that pings do get through, but 
doesn't say anything about port 5900, and nmap, which reports that port 
5900 is 'filtered' but doesn't say by what.

Thanks!

Nigel