The Mother Board

[Karlsweldt]

There are up to 1,000 or more "ports" on a PC setup. Most are virtual, or emphemeral. Some are physical, some are phantom-types.

Here's a list of common ports in use on a PC..

COM port settings: (ISA/AT default)
COM 1= IRQ 4, address 03F8h (normally present)
COM 2= IRQ 3, address 02F8h (normally present)
COM 3= IRQ 4, address 03E8h ('phantom' port)
COM 4= IRQ 3, address 02E8h ('phantom' port)

Parallel Ports: (standard AT)
LPT 1= 0378-037F
LPT 1= 03BC-03BF (older standard, some XT models)
LPT 1 in ECP mode, 0778-077F
LPT 2= 0278-027F
LPT 2 in ECP mode, 0678-067F
LPT 3= 03BC-03BF

[Tramontane]

Here is a short list of ports and their use.

here is a more in depth list of tcp/udp and their like. you can use these lists to figure what is using these ports or for selecting a port for your own use in the event that you may wish to map a port to a particular application.

perhaps someone more intimate with port mapping and firewalls/routers will be kind enough to post after mine.

[Karlsweldt]

Here's some additional info on TCP/IP Port Addressing.
Some more technical info is found here. It is recommended that licensing of the guide be done, instead of simple downloads.

Google can provide many links for "TCP/IP Port Addressing".