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 

ber: 

  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

130.5.0.0/16 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

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