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1.1 | |
VLSM | |
| 1.1.2 | |
A waste of space |
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In the past, it has been recommended that
the first and last subnet not be used. Use of the first subnet, known
as subnet zero, for host addressing was discouraged because of the confusion that can
occur when a network and a subnet have the same addresses. The same
was true with the use of the last subnet, known as the all-ones
subnet. It has
always been true that these subnets could be used. However, it was not
a recommended practice. As networking technologies have evolved, and
IP address depletion has become of real concern, it has become
acceptable practice to use the first and last subnets in a subnetted
network in conjunction with VLSM.
In this network, the network management
team has decided to borrow three bits from the host portion of the Class C
address that has been selected for this addressing scheme.
If management decides to use subnet zero, it has eight useable subnets. Each
may support 30 hosts. If the management decides to use the
no ip subnet-zero
command, it has seven usable subnets with 30 hosts in each subnet. From
Cisco IOS version 12.0, remember that Cisco routers use subnet zero by
default. Therefore Sydney, Brisbane, Perth, and Melbourne remote
offices may each have 30 hosts.
Such an addressing scheme is fine for a
small LAN. However, this addressing scheme is extremely
wasteful if using point-to-point connections.
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The team
realizes that it has to address the three point-to-point WAN links
between Sydney, Brisbane, Perth, and Melbourne. If the team uses the
three remaining subnets for the WAN links, it will have used all of
the available addresses and have no room for growth. The team will
also have wasted the 28 host addresses from each subnet to simply
address three point-to-point networks. Using this addressing scheme
one third of the potential address space will have been wasted. 