HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 661

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring OSPF
An area often contains several subnets. As long as these subnets are contigu-
ous, you can specify all of them at the same time by altering the subnet mask.
For example, in Figure 13-10, area 1 includes two 24-bit subnets, 10.1.4.0 /24
and 10.1.5.0 /24. You could summarize the two networks with 10.1.4.0 /23 and
enter this configuration on ABR A:
ProCurve(config-ospf)# area 1 range 10.1.4.0 255.255.254.0 advertise
You can summarize areas much larger than those shown in Figure 13-10, which
have been minimized for simplicity. For example, to specify a range of net-
works from 192.168.0.0 /24 through 192.168.63.0 /24, you would enter this
command:
ProCurve(config-ospf)# area 1 range 192.168.0.0 255.255.192.0
A quick rule of thumb for specifying a range of class A, B, or C networks is
that the digital number in the last significant octet of the subnet mask is 255
minus the number of subnets available in the range. In the 255.255.192.0 mask
(in CIDR, /18), the first two bits in the third octet are set. The last six bits in
that octet can have any value; therefore, both network 192.168.1.0 /24 and
192.168.63.0 /24 are part of the 192.168.0.0 /18 network. (See Figure 13-11.)
Another way to think of the concept is that every time you subtract one bit
from the prefix length, you double the number of subnets.
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