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  • About This Document

Subnet Sizes Supported

RIPv1 was first implemented when networks were strictly class A, class B, and class C (and later D and E):

This method of address allocation proved to be very inefficient because it provided no flexibility, neither in the way of segmentation (subnetting) or aggregation (supernetting, or CIDR – classless inter-domain routing) by means of VLSM – variable length subnet masks.

VLSM, supported by RIPv2 and OSPF, allows for classless representation of networks to break larger networks into smaller networks:

For example, take the classful 10.0.0.0/8 network, and assign it a /24 netmask. This subnetting allocates an additional 16-bits from the host range to the network range (24-8=16). To calculate the number of additional networks this subnetting provides, raise 2 to the number of additional bits: 2^16=65,536. Thus, rather than having a single network with 16.7 million hosts (usually more than most LAN’s require) it is possible to have 65,536 networks, each with 254 usable hosts.

VLSM also allows for route aggregation (CIDR):

For example, if you had 8 class C networks: 192.168.0.0/24 through 192.168.7.0/24, rather than having to have a separate route statement to each of them, it would be possible to provide a single route to 192.168.0.0/21 which would encompass them all.

This ability, in addition to providing more efficient and flexible allocation of IP address space, also allows routing tables and routing updates to be kept smaller.