For these reasons, RIP 1 is limited to a single subnet mask for each net 
work number. However, there are several advantages to be gained if
more than one subnet mask can be assigned to a given IP network num 
  Multiple subnet masks permit more efficient use of an organization's
assigned IP address space.
  Multiple subnet masks permit route aggregation which can signifi 
cantly reduce the amount of routing information at the backbone
level within an organization's routing domain. 
Efficient Use of Assigned IP Address Space
VLSM supports more efficient use of an organization's assigned IP
address space. The earlier limitation of supporting only a single subnet
mask across a given network prefix locked the organization into a fixed
number of fixed sized subnets. 
For example, assume that a network administrator configured the network with a /22 extended network prefix, as shown in
Figure 16. A /16 network with a /22 extended network prefix would
permit 64 subnets (26), each of which could support a maximum of
1,022 hosts (210 2). 
F I G U R E   1 6 .   1 3 0 . 5 . 0 / 1 6   w i t h   a   / 2 2   E x t e n d e d   N e t w o r k   P re f i x
Please refer to Figure 16. This configuration would be suitable if the
organization wanted to deploy a number of large subnets, but what
about the occasional small subnet containing only 20 or 30 hosts? Since
a subnetted network could have only a single mask, the network admin 
istrator would still be required to assign the 20 or 30 hosts to a subnet
with a 22 bit prefix. This assignment would waste approximately 1,000
IP host addresses for each small subnet deployed. Limiting the associa 
tion of a network number with a single mask did not encourage the
flexible and efficient use of an organization's address space. One solu 
tion to this problem was to allow a subnetted network to be assigned
more than one subnet mask.
U N D E R S TA N D I N G   I P   A D D R E S S I N G
2 0






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