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?