create L2 port on sub interface

[Joli Martinez]

My provider has a /29 and a /30 on their equipment and I dont have access to either equipment.  I have an 1841 router behind their equipment using the /30 on 0/0.  I have created two sub interfaces on the router one for my LAN and one for the second public int, so FA0/1.10 and FA0/1.20.  I would like for 1.20 to be just a L2 interface to get to the providers equipment and pull a public IP from the /29 they are giving me.  If I use one of the public IP's and assign it to the 1841 then point my DMZ devices to the 1841 as a GW it works, but then I am short one IP.  I dont need another public IP on the 1841.

how can I go about doing this without using an extra IP on the 1841?

[Jon Marshall]

I don't think you can do what you want ie. fa0/1 is a L3 port and already has an IP address assigned to it.

From the sounds of it though your provider is routing the additional subnet to the outside interface of your router.

So are you assigning your DMZ devices public IPs directly ?

If so and you want to use all the available IPs can you not just use private IP addressing on the DMZ devices and then use NAT on the router ie. you do not need an interface configured with one of those public IPs as long as your provider is routing the whole subnet to your router.

Jon 

[Joli Martinez]

I was looking at "ip unnumbered" or just not giving the sub interface an IP, but none of those seem to work.  Is there a command that accomplishes what I want without having to use NAT?

[Jon Marshall · Accepted solution]

Can't think of anything.

The issue is your fa0/0 interface already has an IP address and needs one so you need to route to the other subnet.

In which case you either use one of the IPs as the interface IP, get more IPs from your ISP or use NAT.

Is there are specific reason you do not want to use NAT ?

Jon