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You are here: PacificCable.com Restriction: Subnets within networks

Subnets can't span across classful network boundaries. Each subnet must be fully contained with a single classful network. For example, 210.22.74/23 is not a valid subnet, because it spans two class C networks: 210.22.74.0 and 210.22.75.0. On the other hand, 150.22.74/23 is a valid subnet, because it is fully contained within the class B network 150.22.0.0.

Even if your routing protocols fully support CIDR, many operating systems disallow the use of subnets that would span classful networks. For this reason, it's usually a good idea to obey this restriction, even in full CIDR environments. You may get away with assigning 210.22.74/23 today, but later on try to attach a host to that Ethernet that refuses to recognize the 23-bit subnet mask.