HP 600 Series, Services And Applications

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Figure 5. Internet with Variable-Length Subnet Masks
Using the addresses in table C, the internet from figure 4 is shown in figure
5. Full point-to-point link addresses are shown in figure 5 rather than
subnets only, since the subnet mask is split on an octet boundary.
Reserved Addresses
When using variable-length subnet masks, special attention is required to
avoid assigning addresses that are reserved. Avoiding the use of the “all
hosts broadcast” address (all ones assigned to the host-address field of an IP
address) and the “any host” address (all zeros assigned to the host-address
field) was discussed above.
The other consideration that warrants special attention is avoiding the as-
signment of reserved subnet addresses. Reserved subnet addresses are those
that are all ones (subnet broadcast) and those that are all zeros (any subnet).
Variable-length subnet masks introduce extra complexity to this issue, since
there may now be several different subnet definitions in a single network.
For example, consider the internet in figure 5. IP subnets in the range
170.200.0.4 through 170.200.0.248 appear to be valid IP subnets when used
with the WAN subnet mask 255.255.255.252. However, these addresses are
within the range of the reserved LAN subnet addresses, 170.200.0.0 through
170.200.0.255 (subnet mask 255.255.255.0), and thus must not be assigned.
Similarly, subnets 170.200.255.4 through 170.200.255.248 appear to be valid
IP subnets when used with the 255.255.255.252 subnet mask. This address
170.200.1.x 170.200.2.x
170.200.5.1
170.200.5.2
170.200.5.5 170.200.5.14
SUBNET MASK (LAN):
255.255.255.0
SUBNET MASK (WAN):
255.255.255.252
170.200.5.13
170.200.5.10
170.200.5.9
170.200.5.6
170.200.3.x
170.200.4.x
ROUTER
ROUTER
ROUTER
ROUTER
Routing with OSPF
Conserving IP Address Space
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