Workstation cannot reach DHCP server

I have network divided into two segments. Segments are mac-address based and not ip-address based one leased line connect them DSL routers. DHCP server is placed on the end where all other servers are placed and the other segment consist of only fewworkstations. All the workstations in the second segment can reach the DHCP server when they boot and get an IP on lease from DHCP server too accept one machine. Somehow the DSL router replies to its DHCP call and returns its own IP as DHCP, DNS, WINS and bla bla bla… and a unique IP number to workstation by adding one to its own ip number. What could possibly be the reason???