When I first got my xs4all adsl line, it shipped with a Thomson Speedtouch 510 modem.
I liked it, because this modem could be setup to give the external ip to my opendbsd box and all I had to do on my openbsd box was to assign this ip to an interface and all would run fine.

When this modem decided to die, I got a new one from xs4all.
The brandnew 716v with integrated voip and wifi.
This is where the trouble began.

It turned out it could be done. You loose the wifi part of the modem tho.
Here is how I did it.

Let the openbsd server request an ip from the router using DHCP.
As soon as it has one, you can choose "Assign public ip to a device"
Pick your openbsd box here.
Save and reboot your modem.

Now in openbsd setup the "external" interface.
Mine is rl0 and here is my setup:

dhcp
!ifconfig $if 192.168.1.1
!route add default 192.168.1.254
!ifconfig $if description WAN

The !ifconfig and !route entries are needed because openbsd doesn't know how to route the packets.
Thing is you get an public ip on an interface that only has RFC internal gateway.

One last thing to take keep in mind: DON'T do blocking of all rfc networks on external interface in pf.
That will kill your default route.

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