Problems with 10. address scheme

On small network with 12 nodes, all clients running XP Pro, need to change from 192.x address setup to 10.x address setup, for special app to access from outside network.

All connections through Linksys wired router, which serves up addresses through DHCP.

Shared folder on main system is easily accessed when 192.x network is used - with NO other changes in topology, shared folder can no longer be accessed when network number changes to 10.x ...

And when I change address scheme back to 192.x addresses, shared folder is accessible again....

What am I overlooking???? All subnets stay the same, the only change is from 192.x to 10.x -- ?????