Internetwork Packet Exchange (IPX)

The Internetwork Packet Exchange (IPX) protocol was adopted by Novell from the Xerox Network System (XNS) Internet Datagram Protocol. IPX defines:

• Internetwork addressing as the address of a segment on the network, identified by the network number assigned during installation.

• Intranode addressing as the address of a process within a node, identified by a socket number.

IPX is a datagram-based, connectionless protocol. Datagram-based, connectionless protocols do not require an acknowledgment for each packet sent. Packet acknowledgment, or connection control, must be provided by protocols above IPX.

Internetwork addressing

The basis of IPX addressing is the network number assigned during the installation and configuration process. Each network segment on a NetWare internetwork must be assigned a unique network number (sometimes called the external network number). Each server (SCO or NetWare) must also be assigned a unique internal network number. These network numbers are used by routers to forward packets to their final destination segment.

Figure 4-2 illustrates how internal and external network numbers are used.

Figure 4-2 Network numbering

Intranode addressing

The basis of IPX intranode addressing is socket numbers. Since several processes normally operate within a node, socket numbers provide a means by which each process can distinguish itself to IPX. When a process needs to communicate on the network, it requests that a socket number be assigned to it. Any packets IPX receives that are addressed to that socket are passed on to the corresponding process. Hence, socket numbers provide a quick method of routing packets within a node.

Novell has reserved several socket numbers for specific purposes. These are shown in Table 4-3. Because socket numbers are internal to each node, several workstations can use the same socket number at one time without any fear of confusion.

Table 4-3 Socket numbers used in the NetWare environment

 ----------------------------------------------------
 Socket number   Description
 ----------------------------------------------------
 451h            NetWare Core Protocol
 452h            Service Advertising Protocol
 453h            Routing Information Protocol
 455h            NetBIOS
 456h            Diagnostics
 4000h-6000h     Ephemeral sockets; used for
                 interaction with file servers and
                 other network communications

IPX packet structure

The network, node, and socket addresses for both the destination and the source are held within the packet's IPX header. The IPX header is placed after the MAC header and before the packet data. (Packet data usually contains the header of a higher-level protocol.) Figure 4-3 illustrates the structure of an IPX packet on an 802.3 network.

Figure 4-3 IPX packet structure

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