When I checked DHCP snap-in, I found that DHCP releases 10 ip addresses to its own server with RAS as unique id (this server is the domain server, dhcp, dns and RAS server). I was not sure what caused DHCP to release 10 ip addresses with RAS unique id. I sometimes deleted the ip address and didn't affect our operation. I observed for a while and noticed it happened every Monday and about 15:21 weekly. We have a Cisco firewall router and use the router as VPN connection. But, I know no one use VPN in our company except I. I set up WSUS and Time server on this server. If you have any ideas for this cause, I appreciate your comments.