IPX Addressing and Address Structure

IPX is a network layer protocol with its own proprietary addressing structure. This section introduces the IPX address structure that each IPX client (sometimes called a workstation in NetWare documentation) or server must have to communicate with other IPX devices on an internetwork.

An IPX address has two components , a 32-bit network component that applies to a given LAN or WAN segment, and a 48-bit node component that uniquely identifies a client or a server. Most nodes determine this unique number by reading the 48-bit data link layer (Layer 2) address on their LAN interface. As we will see, it is not a requirement of IPX to have the data link layer address of the device match the node component, but by convention, these two numbers often do match.

These two components, expressed together as network.node , are written using hexadecimal format. The two-layer hierarchy of the IPX address structure makes this addressing scheme scalable for internetworks, yet not as scalable as the multiple hierarchies of the IP addressing structure.

The network administrator assigns the network number for an IPX network segment in the same way that he selects IP subnets for given LAN and WAN segments. All IPX clients , IPX servers, and Cisco routers on the same LAN or WAN segment must have the same network number.

NetWare servers have internal IPX network numbers that differ from the IPX network numbers for any LAN or WAN interface. The internal IPX network number is used as the source network number for NetWare services on the server. We'll discuss service advertisement later in this chapter when we discuss SAP. A Cisco router can be configured with an internal IPX network number using the global configuration command ipx internal-network . We discuss this internal network number further in the section "Configuring NLSP."

Each IPX server or client needs to have a unique node number on a LAN or WAN segment. Typically, IPX clients derive this unique number by reading the 48-bit data-link address on their LAN interface and then using that number as their unique network layer node address. Although the LAN interface data-link address is the same as the IPX node address, you should not conclude that a client uses these two addresses in the same manner. The data link layer address is used for data link layer encapsulation, such as with Ethernet or Token Ring. The IPX node address is the second portion of the IPX network layer network.node address for a given client. An IPX client on network 10 with a data-link address of 0802.044d.d88f on its Ethernet interface would be known on the IPX network as 10.0802.044d.d88f by reading the 48-bit data link layer address on its LAN interface.

Using the data link layer address to determine a unique 48-bit IPX node address is not required by the IPX protocol. You can have a node address that does not match a data link layer address, as long as the node address is unique on a given IPX network. For example, we have seen that an IOS device can have multiple LAN interfaces. When IPX routing is enabled, the IOS device chooses the data link layer address on the first LAN interface in the device as the unique node address for all IPX network segments. Now imagine that the data link layer address on Ethernet 0 of a router is 0000.0c11.12ab. If Ethernet 0 is the first LAN interface in this router, and if the router is connected to IPX network 10 and IPX network 20, the router is seen as 10.0000.0c11.12ab on IPX network 10 and 20.0000.0c11.12ab on IPX network 20.

The IOS global configuration command ipx routing enables IPX routing in an IOS device (and enables IPX RIP, which we will discuss later in this chapter). The device automatically chooses an IPX node number based on the first LAN interface when this global configuration command is configured. In the following example, we enable IPX routing on the SF-2 router on the ZIP network:

 SF-2#  configure  Configuring from terminal, memory, or network [terminal]? Enter configuration commands, one per line.  End with CTRL+Z. SF-2(config)#  ipx routing  SF-2(config)#  ^Z  

Note

If your router does not have a LAN interface, you must configure a unique IPX node address as an optional parameter to the ipx routing command. The node address must have 12 decimal digits and must be a unique node number for the IPX networks connected to the router.

The use of the data link layer address to determine the IPX node address simplifies the job of the network administrator, because IPX clients do not need manual configuration. Also, this mapping of data link layer address to network layer address can eliminate the need for a separate protocol to map between the addresses on these two layers , such as ARP. This is discussed in Chapter 4, "TCP/IP Basics."