I need your help!

device (1) is attached to an embedded linux pc (2) through serial port (ppp conection). Linux pc acts as a router attached to lan.
windows xp pc (3) connected through lan to linux box pc
the thing is that I need to reach (1) from (3) with my program.
if I ping from (2) to (1) and from (2) to (3) answers OK.
if I connect (3) to (1) directly through serial port program works ok.
if I try to go from (3) to (1) through (2) IT DOES NOT WORK..
Even having configured linux box serial ports (file ip-up) and lan address (file network.conf and file inittab).

I thank in advance for any hint or help you can give me! I know no linux. OK, just a little but it seems it is not enough...

Thanks again