That's the thing I can't figure out. Which is annoying, because I've managed to figure out everything else so far.

What I've done so far, based on Google searching etc.:

set IP forwarding to 1 in sysctl.conf
typed out 'iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE', and saved it where it should be saved

That's it. I know there's got to be more to it, but damned if I can see it right now (being pretty new to Linux).

It's a dialup connection (hence, the iptables entry referenced above uses 'ppp0' as the external interface). Other than that, there's an eth0 interface, which is on a LAN of course, courtesy of a switch.

The dialup connection works fine (it's what I'm using now), the LAN functions perfectly - communication is clear between this (the gateway) and the switch, this machine and the other PC, and using all other combinations too.

What blindingly obvious thing have I missed?