DHCP on a private address range

I have about 500 laptop and 200 desktop computers and a single C class address. I am trying to set up a DHCP pool in a private address space 172.21.248.0-172.21.251.0 subnet 255.255.252.0 Everything looks ok on a W2K and NT4 server exept that no addresses are leased out. Both of these servers work properly with an address range from the C class range