Problem while pinging

We have 50 systems connected to DHCP which was controlled by Router. Every thing is going on fine with that 50 systems. DNS lookup, can be pinging to lan and wan. We have added one more system to the network (51). as same before just we condigured the system to get ip adr from DHCP. ok. we can ping to any system in lan from that system. but when we are pinging to wan ip adrs. we are not getting results. plz tell me the reason