There are three COM ports: COM1, COM2, COM3 and COM4. Two of these ports are physical (usually COM1 and COM2) and the other two are logical. Windows 95 deals with physical and logical ports much easier than Windows NT 4.0. NT tends to be closely tied to the hardware and thereby more closely tied to COM1 and COM2. For example, my default Windows NT installation only sees COM1 and COM2 in the Ports Control Panel Applet.

Usually, COM1 shares interrupt 4 (IRQ4) with COM3; COM2 shares interrupt 3 (IRQ3) with COM4. Here is the summary of typical COM port addresses and interrupts:

COM1 03F8 - 03FF IRQ4
COM2 02F8 - 02FF IRQ3
COM3 03E8 - 03EF IRQ4
COM4 02E8 - 02EF IRQ3

TIP: Assume that quality motherboards are "smart" enough to handle these port settings for you (whether PNP or not). Leave address settings to "auto" or "default" in the BIOS as much as possible. Don't try to fight the system. Especially one running NT 4.0!

TIP: Here is a summary of typical or traditional interrupt assignments:

00 Timer 
01 Keyboard 
02 Cascade (for interrupts above 8) 
03 COM2, COM4 
04 COM1, COM3 
05 LPT2 
06 Floppy Controller 
07 LPT1 
08 Clock 
09 Sometimes used for ISA in a PCI computer 
12 PS2 Mouse Port 
13 Math Coprocessor 
14 IDE Controller 

Note that interrupts 10 and 11 are generally not used.