How to use Ethereal to find out header signatues for firewalls?

I current have a server acting as a NAT router with ISA 2004 installed.

I need to understand how to use Ethereal to pick up the header name (or whatever) of Ares which is a p2p software. I want to block it.

I know how to use and run ethereal…but i cant seem to find it (header name, signature, etc)

Can u guys please help me, i spent hours searching on google with no luck.

Thanks.