I saw your post in your blog and I am going to use it
tonight. I do
not use 2 ethernet ports although this is probably better. I only have
a DD-WRT router connected to a Speedtouch 585v7 (as an ADSL modem in
bridge mode) and my AMPRnet Gateway in a LinuxMint Box with one
ethernet port and 4 radio ports in it's 4 x RS232 adaptors.
In case your DD-WRT uses PPPoE to "dial" an Internet connection via
eth0, you should have a ppp0 interface which will receive your public
dynamic IP (or if you're unlucky and your ISP employs CGN, you will
receive a private IP and things probably just won't work).
On the DD-WRT ppp0 will be your (virtual) WAN port and eth0 will be
your LAN port.
The eth0 on your DD-WRT and the eth0 on your Linuxbox should be able
to use an IP address within 44/8 without issues. You will just have to
make sure that packets from within your LAN which should go via the
IPIP full mesh must use the LinuxMint Box as default gateway and not
the DD-WRT, while all other boxes (not need for AMPRnet access) and
your LinuxMint box must use the DD-WRT as default gateway.
But you are free to set this up as you prefer.
73 de Marc, LX1DUC