None, sort of. Legacy macs have two serial ports. One is marked for a printer the second for an external modem. By default the printer serial port is used by Appletalk to communicate with a network. You can change the defaults and use the modem port, ethernet, or any other add on communications port.
Now this isnt really as important because almost no one used those serial ports for connecting to a network. In fact new Macs (anything after the Blue & White G3) don't even have serial ports. Appletalk now is usually just routed to use the ethernet port.
So basically the AppleTalk port is just which ever port you designate it to use.