NetBIOS Over IPX Broadcast Packet Structure

The NetBIOS over IPX Broadcast header, shown in Figure 5.17, is a combination of the IPX WAN Broadcast header and a NetBIOS header, which is placed immediately after the IPX header. NetBIOS over IPX Broadcast packets have a Packet Type of 0x14, a Destination Node number of 0xFF-FF-FF-FF-FF-FF, and a Source and Destination Socket number of 0x455.

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Figure 5.17 NetBIOS Over IPX Broadcast Packet Structure

Network 1–Network 8 (IPX WAN Broadcast Header)

The first eight fields (Network 1 through Network 8) are the IPX WAN Broadcast header. The series of IPX networks on which the IPX WAN Broadcast packet has traveled is recorded by IPX routers as the packet is propagated through an IPX internetwork. To prevent loops, this network path information is analyzed upon receipt at a router and the packet is forwarded to all networks except those on which it has already been.

After eight networks, the IPX WAN Broadcast packet is silently discarded by the last router in the path. However, recall that the maximum hop count for any IPX packet on a RIP for IPX–based internetwork is 16. The difference in the maximum number of hops between an IPX WAN Broadcast and a normal IPX packet can cause confusion in large IPX internetworks.

For example, a NetWare server 10 hops away is reachable from a NetWare client because its IPX internetwork address is discovered by querying the SAP table or directory tree of its default NetWare server. Connection requests sent to the NetWare server are unicast traffic that reach the NetWare server because it is within 16 hops.

On the other hand, a Windows 2000 Server–based computer 10 hops away is not reachable for a Microsoft SMB–based client because its IPX internetwork address is discovered using a NetBIOS Name Query sent using a NetBIOS over IPX Broadcast packet that is discarded after eight networks. Because there is no response to the Name Query, the IPX internetwork address of the Windows 2000 Server–based computer is not resolved and a connection cannot be established. To prevent this problem, design your IPX internetwork so that there are no more than seven IPX routers between any two Windows 2000 computers.

Name Type Flags (NetBIOS Header)

The 1-byte Name Type Flags field of the NetBIOS header contains a series of flags to indicate the state of the NetBIOS name. The individual bits are defined in Table 5.5.

Table 5.5 Name Type Flag Bits

Bits are numbered from the high-order bit (Bit 1) to the low-order bit (Bit 8).

Data Stream Type 2 (NetBIOS Header)

The 1-byte Data Stream Type 2 field of the NetBIOS header indicates the type of NetBIOS packet. Table 5.6 lists the defined values for the Data Stream Type 2 field.

Table 5.6 Data Stream Type 2 Values

NetBIOS Name (NetBIOS Header)

The 16-byte NetBIOS Name field of the NetBIOS header stores the NetBIOS name.

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