HP 200 Series Services And Applications page 275

Routing with OSPF
Addressing Flexibility
range is also within the reserved LAN subnet address range and must not be
assigned.
The rule is to avoid assigning addresses within the reserved address ranges
given by the subnet mask with the fewest number of bits in the subnet ID
field. In figure 5 the subnet mask with the fewest number of bits in the
subnet ID is 255.255.255.0. Thus the reserved address ranges are 170.200.0.0
through 170.200.0.255 and 170.200.255.0 through 170.200.255.255.
Addressing Flexibility
OSPF enhances IP addressing flexibility by allowing IP networks to be parti-
tioned. Network partitioning is not permissible with RIP. A network becomes
partitioned when one or more subnets of a network become separated from
the other subnets of the same network by a second network. In figure 6, the
class B network 128.1.x.x is partitioned by network 192.1.1.x.
128.1.2.x
128.1.1.x
192.1.1.x
ROUTER
ROUTER
Figure 6. A Partitioned Network
To understand why network partitioning might be useful, refer again to
figure 6. Suppose network 128.1.x.x is a large network with very few
remaining subnets. Subnets of the class C network 192.1.1.x can be used for
synchronous links so that none of the remaining class B subnets have to be
used to extend the network.
Another situation in which support for partitioned networks is helpful is
when networks are combined as a result of a merger or acquisition.
Networks often overlap in this case, often resulting in capacity imbalances.
Being able to partition a network allows the network's administrators much
more flexibility in assembling the combined networks.
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