Many DSL companies have settings in their modems that affect your NAT. Century Link and their Zyxel(?) 660 modem is famous for this. There are a couple of settings you can change in the modem setup to make it a dumb modem and remove any restrictions - try googling your modem name if you have DSL.
ok ok..... i just ended up forgetting it as we just played in a part the whole weekend, but yeah it is a centurylink 660..........( this was at a friends), i had one here, and i was replacing it once a week.....finally upgraded to a 7500 westell (modem/router/wifi/switch) havent had a single problem