Brian Densmore wrote:

> netmask is 172.21.12.255 which is correct.

It is?  Subnet masks are nearly always a string of ones, followed by a
string of zeroes, e.g. "255.255.255.0".  The ones indicate which bits in
an address designate the subnet part, and the zeroes indicate which bits
in an address designate the host part.  "172.21.12.255" looks an awful
lot like a broadcast address on a network with a 24-bit netmask.