Need help!! Interface answer to public address but not to private address

Hello,

I have a Cisco Router 3725 with a Network Module with an extra FastEthernet card. So it has 3 FastEthernet interfaces in total.

I have a PC connected to one of them (the one in the Network Module) with a cross-cable. If I configure a public ip in the interface and in the PC I am able to ping to the router interface, but if I configure a private IP I am not able. However in the last case debugging with "debugg ip packet" I can see as the ping packet arrives and it shows that the ping reply packet leaves the router. However it doen't arrive to the PC. The physical cable is ok as using public address it works.

In the last case, if pinging to the other two interfaces they answer if they have public ip configured if not they not responf.

If connecting the PC to the other two interfaces I am able to ping all the interfaces of the router, even the interface with problems.

The think is two days before it was working and I have updated the router to the backup I had from two days before...

Any idea?

Thanks, Marcos