Like TCP/IP and AppleTalk, IPX/SPX supports a 32-bit address, which is usually expressed in hexadecimal, as in 0x23a91002. This address, however, isn't assigned to a single computer, but to an entire network—usually a network segment that's isolated from others by routers, or completely disconnected from the outside world. An IPX/SPX network is also identified by the underlying hardware's frame type, which is how the Ethernet frames are built at a very low level; all computers on a single IPX/SPX network must use the same frame type. To identify individual computers, IPX/SPX relies on the underlying hardware's addressing scheme, such as Ethernet's 48-bit (6-byte) addresses.
As you might guess from the name and addressing scheme, IPX/SPX is designed for internetworking—that is, linking networks together. This is accomplished via IPX routers, which work much like TCP/IP routers in broad detail. (In fact, a single system can function as both a TCP/IP and an IPX router.) A simple network may not require a router, but IPX/SPX does support the option.
IPX/SPX servers use a protocol known as the Service Advertisement Protocol (SAP) to periodically announce their names and the services they make available, such as shared directories or printers. Other systems on the local network segment will "hear" these announcements, and IPX routers should echo them to other network segments. This design can help make locating the right server easy, but it can also increase network traffic as the network size increases; with more servers, there will be more SAP broadcasts consuming network bandwidth.
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