Serial ports are most often used on networking devices to provide out of band access for preconfiguration of various switches and routers before use. They are also often used to provide emergency access in case of a loss of connectivity to the device, and to provide troubleshooting, as many consoles spam error messages across the screen that would otherwise not be visible (except in logs), via SSH or other connection methods.
Adapters connecting to a device's console port are often a DB-9,25-pin D-sub connection, or a CAT-5/CAT6 RJ-45 port; in the case of the DB-9 or 25-pin D-sub (colloquially an "RS-232 adapter"), this is usually an adapter with the aformentioned RJ-45-terminated cable coming out the other end for connection to a console router through which out of band devices may be accessed. Ethernet consoles typically plug right into the device itself.
A more common term for serial port access is console, in fact: this term has come to mean not only the direct serial console for switches but also console ports on servers. Console ports are usually accessible via telnet - modern consoles, however, provide SSH and even graphical interaction with the console.
- Sun uses Lights Out Management, often shortened to just LOM. This provides remote access to the server, power on, power off functionality, and a few environmental controls.
- HP uses the iLo, and more recently, the iLo2. This is not, strictly speaking, a serial port, but provides roughly the analogue to it, allowing power on, power off, and access to the shell.
- Dell uses both IPMI and DRAC, a proprietary implementation of IPMI.
- IPMI is a non-manufacturer specific console that may be dropped into just about any server. The most common tool used with this is ipmitool.