SuSE 10.0 masquerade changes?

I got simple masquerading working under SuSE 9.3 (sharing a modem), but I can't get it working under SuSE 10.0 . I can ping and ftp within my internal network, but the internal network can't see the internet. Has anything relevant changed between 9.3 and 10.0, or am I doing something stupid? Any ideas? Where do I look for clues? Peter Taylor