Summary:

A network server requires a network interface. On most servers, including Linux systems, this is an Ethernet interface. However, Linux systems also support a full range of network services through the serial port. Connect a terminal or modem to a Linux serial port, and it will support user logins. Run the Point-to-Point Protocol (PPP), and the serial port can be used to provide TCP/IP connections.

The same flexibility that Linux shows in the types of network connections it provides is seen in the full range of network services offered by a Linux system.