This is not a firewall issue.
I have an ADSL connection, and have set port 50420 to be forwarded locally for use with utorrent. Everything was working fine, and various tests and utorrent itself shows the port as being forwarded correctly, as well as various other client programs.
However, after sharing my internet(LAN) connection via my wireless connection, the port stops being forwarded. Nothing gets changed on the modem, so I am unsure as to why this is happening.
My Ethernet adapter has DHCP enabled, and gets an addresses automatically from the modem. Sharing this connection assigns my wireless connection a static IP of 192.168.0.1 and allows for clients to use my computer as a gateway.
I am unsure why this prevents the port from being forwarded correctly....