i use a dsl connection at work, and I've used it with and without a dsl router.
without the router, the dsl modem actually gave my nic an ip via dhcp. the address was like 192.168.1.11 and the dsl modem was using 192.168.1.1. Then a ppp connection was established over the ethernet connection (known as PPPoE). This is the ppp0 connection that you have, and it gets the real internet ip address.
for your setup i don't think you need to run a dhcp server, that's overkill. you can just use static ip's.
andre = 192.168.1.5 , connected to eth1(192.168.1.4) over crossover cables
julian = 192.168.1.7, connected to eth2(192.168.1.6) over crossover cables
ifconfig eth1 192.168.1.4
ifconfig eth2 192.168.1.6
you need to modify your routing so that you can connnect to each of these boxes. You'll want it to look something like
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
126.96.36.199 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
192.168.1.5 0.0.0.0 255.255.255.255 U 0 0 0 eth1
192.168.1.7 0.0.0.0 255.255.255.255 U 0 0 0 eth2
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 188.8.131.52 0.0.0.0 UG 0 0 0 ppp0
so you'll have to delete any other routes that go to eth1 and to and add the ones above
route add 192.168.1.5 netmask 255.255.255.255 eth1
route add 192.168.1.7 netmask 255.255.255.255 eth2
andre should be able to ping 192.168.1.4 and julian should be able to ping 192.168.1.6
you said that pinging the other machines was actually working before, but i'm not quite sure how. the routing didn't seem correct to me.
the adsl-start script should setup the routing for you if you chose that option in the adsl-setup script. It probably does a couple of things. 1st it probably sets FORWARD=1, so that now your linux box can act as a gateway. 2nd it probably added a few rules for forwarding requests.
I'm not sure how elaborate the forwarding rules are since I haven't used forwardig with this script. You can view how the rules are setup with 'iptables -L' which will list the rules for each chain.
If you want to know more about iptables (you said you wanted to learn linux), you can find out more at http://www.tldp.org.