System information

The Network Layer
Book Title
11-252
Addressing
DECnet addresses are not associated with the physical networks to which the nodes are connected.
Instead, DECnet locates hosts using area/node address pairs. An area’s value ranges from 1 to 63,
inclusive. A node address can be between 1 and 1,023, inclusive. Therefore, each area can have 1,023
nodes, and approximately 65,000 nodes can be addressed in a DECnet network. Areas can span
many routers, and a single cable can support many areas. Therefore, if a node has several network
interfaces, it uses the same area/node address for each interface. Figure 11-4 shows a sample DECnet
network with several addressable entities.
Figure 11-4 Examples of DECnet Addresses
DECnet hosts do not use manufacturer-assigned Media Access Control (MAC)-layer addresses.
Instead, network-level addresses are embedded in the MAC-layer address according to an algorithm
that multiplies the area number by 1,024 and adds the node number to the product. The resulting
16-bit decimal address is converted to a hexadecimal number and appended to the address
AA00.0400 in byte-swapped order, with the least significant byte first. For example, DECnet address
12.75 becomes 12363 (base 10), which equals 304B (base 16). After this byte-swapped address is
appended to the standard DECnet MAC address prefix, the resulting address is AA00.0400.4B30.
Routing Levels
DECnet routing nodes are referred to as either Level 1 or Level 2 routers. A Level 1 router
communicates with end nodes and with other Level 1 routers in a particular area. Level 2 routers
communicate with Level 1 routers in the same area and with Level 2 routers in different areas.
Together, Level 1 and Level 2 routers form a hierarchical routing scheme. This relationship is
illustrated in Figure 11-5.
10
Area
number
1
Node
number
10 1
10 3
10 2
Area 10
5 1
5 3
5 2
Area 5