Port blocked prevents app from working

Question

Dear all,
Although I don't even have ipfilters support compiled into my kernel, external attempts to connect to my tcp port 4662 fail. This happens although (locally) nmap shows it to be open:
# nmap -sT -P0 -p 1-65535 localhost
Starting nmap 3.81 ( http://www.insecure.org/nmap/ ) at 2005-05-03 23:40 EDT
Interesting ports on thinkpad (127.0.0.1):
(The 65533 ports scanned but not shown below are in state: closed)
PORT     STATE SERVICE
4662/tcp open  unknown
6000/tcp open  X11
Nmap finished: 1 IP address (1 host up) scanned in 2.805 seconds
However, it is impossible to connect from the internet to this port. Eg, I've used
http://www.amule.org/testport.php?tcpport=4662
and it always answers that the port is inacessible.
Do you have any idea why this port might not be accessible on my machine? I am a residential cable customer, so I'm not behind a firewall.
Thanks for any clue,
Mack

Answer 1

Actually a cable modem, acts as a router, as long as it has DHCP service in it, and im pretty sure its in NAT mode (so if you connect a switch to it, you can access internet from multiple PCs)...
Arch has no port blocking per default that im aware of.
EDIT:
Also, as you see with nmap the port is open in your computer, but you cant reach it from internet because when you are behind a NAT you have to forward that port in the router/cablemodem/dslmodem to your PC IP and port. This was what i was talking in my first post. (Im quite sure about the cablemodem being in NAT mode and causing this problem, but i could be wrong, in my case, this was the problem (though i have DSL)).