I recently reinstalled my Rialo application server on a Windows 7 Home Premium PC. I changed the port in server.xml from 8080 to 80 but Tomcat no longer would serve pages.

After about an hour I found a forum post that blamed Skype. Sure enough I stopped Skype and Tomcat now runs.

c:\ netstat -abn -p -tcp

showed that Skype was “ESTABLISHED” and blocking the port.

My colleague Chris says

“What happens is that the OUTGOING request goes to a well-known-port

80 is http, 443 https, 22 telnet/ssh etc

There is a pretty fixed list.

Once the outgoing gets a response, there is an immediate hand off; each side uses a new, and agreed, port; which leaves the request port open for the next request.

Now, if a firewall shuts down all INCOMING ports then we’re in trouble. If only 80 is open, then only one service can deal with it. And Skype will grab it if it can. So port 80 does Skype service.

If it shuts down all OUTGOING ports then you get some congestion – but Skype will go upstream via port 80 if it has to. Tunnelling, that is, but has small performance effects.

[… fix your Windows firewall]”