Most machines either have no serial ports, or one DB9 serial port. Details of other configurations are listed here.
taurine has a virtual serial port that can be accessed via telnet or SSH
ttyS0 physical DB9 ttyS1 virtual
ascorbic-acid has two Sun 8P8C serial ports stacked vertically, with the first port on top
A (ttyS0, LOM) B (ttyS1)
These ports should be connected to another system's DB9 using a DB9<->8P8C modular jack and an Almost Rollover or standard rollover.
potassium-citrate has two DB9 serial ports side by side, with the second port on the left
101102 (ttyS1) 101101 (ttyS0)
citric-acid has two DB9 serial ports stacked vertically, with the second port on the bottom, and a dead DB9 labeled "modem" to its right
ttyS0 X ttyS1 modem