Routing problem

We are setting up a pilot lab in our office for training purposes.We have two windows 2003 server sp1 and both are different subnet masks.we used an internet router to interconnect both the LAN.After configuring the router we found that we can ping from LAN(Ist network) to WAN(IInd network) .But we could not able to ping from WAN to LAN.Can any one help us to overcome the problem..please