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OSI Layer 3: Routed and Routing Protocols
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We start out with the 16-bit Checksum field with a default setting of
0xFFFF. This field is followed by the 16-bit Packet length field. The length
field indicates the complete length, in bytes, of the IPX packet, header, and
payload.
The next two fields (Transport control and Packet type) are eight
bits each. Transport control is a counter that counts the number of rout-
ers a packet has transited. Packet type indicates which of the upper-layer
protocols are to receive the PDU after Layer 4 is through processing it.
The fourth layer contains the Destination IPX network field, a 32-bit
field that contains the IPX network for the destination host. The fifth layer
is the Destination node. This field is 48 bits long and contains the MAC
address of the destination host. The Destination socket is a 16-bit field
on the sixth layer of the IPX header. This identifies the socket number for the
upper layers.
Following the destination information, three more layers contain the
same type of information for the source host. The seventh layer is the Source
IPX Network
, another 32-bit field containing the source network. This is
followed by the Source node, a 48-bit field containing the MAC address of
the source host. Finally, the last field of the header contains source host
socket information, the Source socket field, which is 16 bits in length.
Certain codes are used to specify which socket numbers are assigned to
which protocols. The codes used are shown in Table 2.13.
T A B L E 2 . 1 3
IPX Socket Number Assignments
Socket Number
Upper-Layer Protocol
0451
NetWare Core Protocol
0452
SAP (Service Advertising Protocol)
0453
RIP
0455
NetBIOS
0456
Diagnostics
4000-6000
Temporary file server and network communications
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