Sanity check.

I have customer in a very remote location with a DSL router/modem with a /30 subnet assigned to it's WAN interface. The upper of the two available IP addresses is used for the router and the lower for the ISP's gateway on the other side of the DSL link. The LAN network is on a private range using NAT.

I want to change the router/modem to use only public IP addresses and have another Ethernet router behind that router which has more features (i.e. IPSec etc.) and put the private network on the LAN port of the router.

I've come to do so and I now think it is impossible because a /30 does not give me enough IP addresses. Is this correct?

  • It seems this isn't possible as I suspected. Aug 8, 2015 at 12:09
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 12, 2017 at 3:41

2 Answers 2


Connect the expensive router to the DSL modem and switch all routing as well as internet connectivity setup off on the router/modem. Then the modem will not use any IP. Most devices (home routers) provide such features, often called PPPoE pass-through.

Then let the expensive router establish the PPPoE connection to the ISP via the external modem. If you provide more details about the involved devices, we might figure out more details.

  • Yes, I've done this in the past in UK. You use a router that supports PPPoA to PPPoE and then you put the username and the password. This router is 4,500 miles away and uses 1483 encapsulation. I expect I could get it to work but not without an expensive flight. Jul 28, 2015 at 16:30
  • 2
    Remote hands? Cheaper? Not sure what your problem is exactly. I mean you just don't have another IP in that /30. You need to take it away from the router/modem. Jul 28, 2015 at 16:33
  • Yup, just a sanity check as I say. I'll see if I can send out a pre-configured DSL modem with bridging, but this is in a developing country and nothing is straight forward. Jul 28, 2015 at 16:56

If the DSL modem doesn't provide PPPoE pass-trough but does have a "DMZ" option, I.E. forwarding all incoming traffic to an internal IP, you can also use double-NAT.

You set a fixed private IP on the WAN interface of the new router and forward all incoming traffic to it, and set another private network on the LAN side.

The new router will NAT all traffic with it's WAN private IP then forward it to the DSL router which will NAT it again with the public IP.

Like : network -> ( - New router - -> ( - DSL router - public IP) -> the Internet

Note that IPSEC doesn't work well with NAT, if you need it you have to enable ipsec NAT traversal. (or you may use another kind of VPN like OpenVPN)

  • I though of double NAT but I needed IPSec Aug 8, 2015 at 12:08
  • IPSEC can work with double NAT, with ipsec nat traversal (I use it on several places) but is a little bit more tricky to set up, depending on which endpoints you use.
    – JFL
    Aug 10, 2015 at 7:51

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