Can't use P2P clients... Please help!

Okay so here's my problem... I just set myself up with cable internet at home using a Motorola SBG900 modem with a built in router. Everything was working great until I tried to start up a P2P client, when I quickly realized no P2P client would even connect at all.

I logged in to my router to mess around with some settings and see if I could get things working, and after some trial and error, noticed that when I set my firewall policy to "Low" or "None" I am suddenly able to connect through any P2P client I want. So I started Googling around and found some information on things like "speeding up your BitTorrent downloads" and such, and how one should change the "port range forwarding" when logged in to one's router... so I started digging around again in my router's settings and came up short. No "port range forwarding", no "BitTorrent" port settings, no "P2P" settings... I just don't know what to do.

By the way, I am a strong supporter of legal downloads... I do a lot of video editinguploading/downloading and only use P2P clients for legal files, mostly video transfers and such.

My question may be more basic than I am making it out to be... what I really want to know is, what is the Port ID that BitTorrent uses? I have a whole list of Port ID's (such as SNMP, DHCP, DNS, TFTP, Syslog, ICMP, RSVP, IGMP, etc. etc. on and on) listed when logged in to my router, and I'm just not sure which one pertains to BitTorrent transfers.

Any help is greatly appreciated. Thanks so much, in advance.