how do i open ports for xbox live using Ethernet connection with computer ?

[adamsedixon87]

Hello.I have an internet connection using an AT&T usb internet card or watever it is.but i connect through it. i had to install it to my computer for it to work it connects GSM or something like dat. This the only "high speed" connection that available for me since i live in a rural area. I have my xbox connected to my computer via ethernet and sharing a connection with AT&T usb device. wen i connect to xbox live it says that my NAT isnt open or something.I been reading instructions all week about port forwarding but all of them explains with a router. i also been calling xbox live and my isp but its not helpin. I been tryin to figure it out by trial and error. i believe opened the ports but i didn't do something rite if not anything. i know wat ports need to be opened or forwarded. PLEASE HELP me i really want to play on xbox live. PS(one port i need to open is tcp 80 but it is bein used by the web server and wont let me change it because the area where u enter the text is faded.) PLEASE HELP!!!