Host Implications for CIDR Deployment

It is important to note that there may be severe host implications when you deploy CIDR based networks. Since many hosts are classful, their user interface will not permit them to be configured with a mask that is shorter than the "natural" mask for a traditional classful address. For example, potential problems could exist if you wanted to deploy 200.25.16.0 as a /20 to define a network capable of supporting 4,094 (2 12 -2) hosts. The software executing on each end station might not allow a traditional Class C (200.25.16.0) to be configured with a 20-bit mask since the natural mask for a Class C network is a 24-bit mask. If the host software supports CIDR, it will permit shorter masks to be configured.

However, there will be no host problems if you were to deploy the 200.25.16.0/20 (a traditional Class C) allocation as a block of 16 /24s since non-CIDR hosts will interpret their local /24 as a Class C. Likewise, 130.14.0.0/16 (a traditional Class B) could be deployed as a block of 255 /24s since the hosts will interpret the /24s as subnets of a /16. If host software supports the configuration of shorter than expected masks, the network manager has tremendous flexibility in network design and address allocation.