Symptoms: In your domain network, some computers (most are win9x) obtain mask 255.255.255.0 instead of 255.0.0.0 randomly and they can't logon to the Domain. IPCONFIG /renew doesn't fix the problem. If you assign static ip and correct mask, the computer will be able to logon without any problem. If you check the WINS, you may find many bad records.
Possible reasons: you may have another network device (possible a router) except main DHCP functions as a DHCP.