Linux PPP HOWTO
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# 24.2. Setting up the routing

You must arrange that packets on your local LAN are routed across the interface that the PPP link establishes. This is a two stage process.

First of all, you need to establish a route from the machine running the PPP link to the network(s) at the far end of the link. If the link is to the Internet, this can be handled by a default route established by pppd itself at your end of the connection using the 'defaultroute' option to pppd.

If however, the link is only linking two LANs, then a specific network route must be added for each network that is accessible across the link. This is done using a 'route' command for each network in the /etc/ppp/ip-up script (see "After the link comes up"...) for instructions on doing this.

The second thing you need to do is to tell the other computers on your LAN that your Linux computer is actually the 'gateway' for the network(s) at the far end of the ppp link.

Of course, the network administrator at the other end of the link has to do all this too! However, as s/he will be routing packets to your specific networks, a specific network route will be required, not a default route (unless the LANs at the far and of the link are linking into you to access the Internet across your connection).

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