One of the advantages of Linux (UNIX) over some other operating systems is that it can be maintained on the command line, and hence is useful for accessing over a modem or serial connection (e.g. via a Baytech or Terminal Server device).
The problem is that there are so many different standards for RS232 cabling, which was traditionally on a DB9 or DB25 connector, although is increasingly on RJ45 connectors.
It’s not just different connectors, but then different types of cable, e.g. straight; crossover (null modem); or rollover (a Cisco thing that has now become more common with other manufacturers).
Here is a useful resource to try and make it a little easier to understand: