with ciscos, you can use subnets all ones and all zeroes.
We recently changed (a year or so ago) to allow use of the all ones.
We still make you turn a knob (ip subnet-zero) before allowing you to
use subnet zero since some hosts and some vendor's implementations of
maskless routing protocols barf when you use subnet zero.
The current router requirements now specifies that the all zeros and all
ones subnets are totally legal.