ichat video chat

just recently my family got an iMac and ive had a macbook for a while now, but never did video chat. i have just switched to FIOS (just putting it here in case it helps get a specific answer) and now trying with the macbook video chatting with a PC user is fine. however on the imac which is wired to router it does not allow me to connect. does anyone know a specific set of ports i should maybe try opening? or just a strait solution to this problem?

i had a similar problem, iMac to iMac and had to turn on wireless and use that vs the direct wire. there was a compatibility issue with the router I was using and the router my cousin was using. if you don't have wireless have the other end try it.