[Solved thanks to Hu and Mokia] Simple routing question.

[ispano]

And seeing as the command you posted last bypasses the rule for that ip, I think I'll use that.

[mokia]

use this:
-I FORWARD -d 192.168.100.1/32 -i eth1 -j ACCEPT
It allows the modem connection. But dint open your hole internal network for crackers.

Afther this rule
iptables-save looks like this:

# Generated by iptables-save v1.4.6 on Sun Jul 18 07:03:12 2010
*filter
:FORWARD DROP [0:0]
-A FORWARD -d 192.168.100.1/32 -i eth1 -j ACCEPT <-this must be on on top
-A FORWARD -d 192.168.0.0/16 -i eth1 -j DROP
-A FORWARD -s 192.168.0.0/16 -i eth1 -j ACCEPT
-A FORWARD -d 192.168.0.0/16 -i eth0 -j ACCEPT
COMMIT
# Completed on Sun Jul 18 07:03:12 2010

EDIT
Sorry for the forward

[ispano]

Yeah I did use it, and it works fine. Looked at it after temporarily disabling "-A FORWARD -d 192.168.0.0/16 -i eth1 -j DROP" and it clicked what it did, so I re-enabled "-A FORWARD -d 192.168.0.0/16 -i eth1 -j DROP" and used your command.

Looks like this now:
-A FORWARD -d 192.168.100.1/32 -i eth1 -j ACCEPT
-A FORWARD -d 192.168.0.0/16 -i eth1 -j DROP
-A FORWARD -s 192.168.0.0/16 -i eth1 -j ACCEPT
-A FORWARD -d 192.168.0.0/16 -i eth0 -j ACCEPT
COMMIT
# Completed on Sun Jul 18 08:37:37 2010

I learn something new everyday with Gentoo, always a good thing.

Thanks again.

[Hu]

Normally, you would use that rule. Normally, it would work fine. Unfortunately, someone had the bright idea of putting an RFC1918 reserved address on a device designed to be on the Internet side of your router.

[mokia]

I think this is the solution, for the RFC standard problem:
-I FORWARD -s 10.0.1.1/32 -i eth0 -j DROP
-I FORWARD -d 10.0.1.1/32 -i eth0 -j DROP