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IPX/SPX (Internetwork Packet Exchange/Sequenced Packet Exchange)

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IPX/SPX is the legacy internetworking protocol for Novell NetWare. It was derived from the XNS (Xerox Network System) protocol, which was developed in the 1970s. It is usually just called "IPX," although some sources refer to it as "IPX/SPX," "SPX/IPX," or "Novell protocol." Note that TCP/IP is now the primary Novell NetWare internetwork protocol.

Currently, a number of other network operating systems, including Windows NT, include IPX protocol stacks. The protocol is an easy-to-configure internetworking protocol, suitable for small networks, and it provides compatibility with legacy Novell NetWare networks. A number of network clients also support the protocol, including Microsoft Windows.

IPX provides datagram services over packet-switched internetworks. Its basic operation is similar to IP (Internet Protocol), but its addressing scheme, packet structure, and general scope are different. Internetworking protocols operate in the network layer and include routing services, as shown in Figure I-15.

The other member of the Novell NetWare protocol suite is SPX (Sequenced Packet Exchange), which resides in the transport layer. When compared to the TCP/IP protocol suite, IPX provides routing and internetwork services similar to IP, and SPX provides transport layer services similar to TCP. IPX and IP are connectionless datagram protocols, while SPX and TCP are connection-oriented protocols. See "Connection-Oriented and Connectionless Services" for additional information.

IPX addresses include a network address and a node address. Network addresses are assigned when setting up the primary server on a NetWare LAN. The node address is the hardwired address on a network interface card. A complete IPX address is a 12-byte hexadecimal number that may look similar to the following, where the first part is the network address and the second part is the hardwired node address:

4A87B321 14594EA221AE 0119

Two routing protocols are available in traditional NetWare environments: RIP (Routing Information Protocol) and NLSP (NetWare Link Services Protocol). RIP is the traditional routing protocol for NetWare. The preferred protocol is NLSP, which uses more efficient link-state routing algorithms. Link-state routing protocols track the status of other routers and links, and can adapt more quickly to changes in network topology. These protocols are discussed elsewhere.

Servers and routers on NetWare networks use SAP (Service Advertising Protocol) to broadcast a message that indicates the types of services they provide. These messages are broadcast every 60 seconds. SAP is similar to RIP in that it enables network devices to exchange information about their availability on the network. However, SAP broadcasting can add unnecessary traffic to networks. To reduce SAP broadcasts, you can increase the interval at which they occur or use Novell-provided filters that reduce SAP traffic over wide area links.

NCP (NetWare Core Protocol) is the principal protocol for handling service requests between NetWare servers and clients. NCP handles logon requests, and many other types of requests to the file system and the printing system. IPX is the underlying protocol that carries the transmission. NCP is a LAN protocol that was originally designed with the assumption that servers and workstations would be relatively close. When a router gets involved and connections are made over wide area network links, NCP causes traffic congestion. It uses a request/response scheme to manage server/workstation communication. If a workstation makes a request, it must first wait for a response from the server before making another request. This required acknowledgment adds excess traffic.

This topic continues in "The Encyclopedia of Networking and Telecommunications."

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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