What does it do?
From the earliest Macs, networking ports have been built-in. The oldest have round ‘mini DIN’ connectors for ‘LocalTalk’, still supported by some printers. Recent models offer one of several types of Ethernet port: older versions may have a round BNC coaxial connector for thin Ethernet cabling, or a special D socket for an adaptor (AAUI). Current models have phone-style sockets for unshielded twisted pair (UTP) cabling to a network hub. The port allows you to print to network printers and print servers, and to share files and other facilities with servers and other computers on the network. How does it work?
The network port provides electrical connectivity for networking software, which runs a protocol such as AppleTalk or TCP/IP in the messages exchanged with the network. Electrically the simplest, thin or coaxial Ethernet (10Base2) sends the data along a single wire in the core of the cable, using the sheath as the ground. LocalTalk used different protocols over more wires, running at around 1% of the speed of Ethernet. AAUI adaptors are available for both thin Ethernet and modern UTP cabling. Although more flexible they perhaps revealed how Apple was unsure as to the best way ahead. Modern UTP systems can support all three network speeds, of 10Mbit/sec (10Base-T), 100 (100Base-T) and the latest 1Gbit/sec (1000Base-T). What can go wrong with it?
Problems normally stem from software issues, or from electrical disruption to the network itself. Unfortunately, these can be hard to diagnose: snags normally occur when incorrectly set up, for example, in the AppleTalk and TCP/IP control panels (Network and MacTCP on older systems). Thin Ethernet cabling is particularly prone to electrical problems, and must be checked carefully for continuity and termination. An engineer can be essential. How can I upgrade it?
For most models, the only upgrade option is to install an expansion card with support for the newer/faster network connection. Indeed, this can be the only way to obtain anything faster than LocalTalk. These may need software drivers, and/or Mac OS upgrades. Further info
Web sites with further information include: Farallon’s fine help pages at www.farallon.com/homenet. Threemacs.com’s guide to small networks at threemacs.com/network/index.html. Older Mac info (including MacTCP, and so on) at www.louisville.edu/~lmlars01/indulge. | 
