HP 200 Series Services And Applications page 272

Routing with OSPF
Conserving IP Address Space
170.200.1.x
170.200.2.x
170.200.5.x
ROUTER
ROUTER
170.200.8.x
SUBNET MASK:
170.200.6.x
255.255.255.0
ROUTER
ROUTER
170.200.7.x
170.200.4.x
170.200.3.x
Figure 4. Subnetting in a RIP Internet
The internet in figure 4 uses the IP class B address 170.200.x.x. This internet
is subnetted using the subnet mask 255.255.255.0. There are 254 possible
addresses on each subnet (addresses 0 and 255 are reserved). On each of the
wide-area network subnets, however, there are only two members, namely
the router on each end of the point-to-point link. Of the 254 possible (or
allocated) addresses, only two will be used. Thus 252 addresses are unused
or wasted. Of the approximately 2000 IP addresses allocated in the internet
in figure 4, roughly half are unusable because they are allocated to point-to-
point networks. Variable-length subnet masks, a feature of OSPF, can be
used to minimize the allocation of IP addresses, such as in the case of the
point-to-point links in figure 4.
Variable-Length Subnet Masks
The mechanics of conserving IP address space using variable-length subnet
masks are straightforward. Again consider the internet in figure 4. With
OSPF the subnet mask can be specified per network or subnetwork. The
subnet masks for the LAN subnets can be specified as before (255.255.255.0).
This allows 254 addressable nodes per subnet. The point-to-point links,
however, require only two IP addresses—one for each router. To restrict the
number of addresses on the point-to-point links, a subnet mask is needed
that allows only two addresses. It is tempting to use the subnet mask
255.255.255.254. The two addresses that this mask provides, however, are
the "broadcast" address and the "any host" address for the subnet. There-
fore, one additional bit is required in the subnet mask, so that reserved
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