Win2000 DHCP Problem

Setup DHCP on Win2000.
A super region including two DHCP region.
All my win2000 workstations works well,but some of my win98 clients get wrong subnet mask as 255.0.0.0,I have to run ipconfig/release_all & ipconfig/renew_all to refresh his IP.then,everything comes to be ok.
Why?
any assistance will be appreciated.