A communications port is a serial port on a computer. Data traveling through a serial port will be transferred single file, one bit at a time (i.e. in a series). Serial ports are rather like a two-lane highway. Data can only travel one bit (one car) at a time in one direction. The following is a list of available COM Ports and their default resources: Port Address IRQ COM 1 03F8 COM 2 02F8 COM 3 03E8 COM 4 02E8 As you can see, COM1 and COM3 share an IRQ as do COM2 and COM4. Most systems have the mouse configured to COM1, the modem to COM2, and do not have any devices assigned to COM3 and COM4. With this configuration, the mouse and modem can work together without conflicts. However, if devices are added to COM3 and COM4, it may be advantageous to manually configure their IRQs in order to avoid conflicts with COM1 and COM2.