Dynamic IP and Static IP on same Dialer Interface

Hi All,

I'm having a bit of an hard time trying to implement a configuration that I find wierd that an ISP provides it that way to start with.

Here is the deal.

I have this ADSL service on a Cisco 836. Initially it was a simple ADSL service with a fixed IP obtained via DHCP (ip address negotiated). I had this IP address NATed with a 'ip nat inside source static tcp....' - everything ok

Latter we agrred with the ISP another static IP address, delivered in the same way, I just had to add a new set of 'ip nat inside source static tcp ..' for the other external address and eveything was still ok.

Now, we recently asked our ISP for two extra addresses, but the ISP said that those addresses could no longer be provided in the same manner as the previuous ones and they would offer an IP pool of 4 addresses in a subneted /30 network.

The trouble is.. those addreses are not obtainable with the 'ip address negotiated' option, so I believe that they are not in the ISP DHCP pool.

Contacting ISP support, they told me that I had to manualy configure the interface for the /30 network and route the traffic instead of nating it. The routing issue I could circunvent.. if I had the addreses delivered in my router together with the other two that are DHCP negotiated.

Can I have both setups in the same dialer? i.e an ip address negociated and a static IP configuration. I know I can't do this directly, but it there any trick I can use.. such as creating another interface and bridge it or something alike?

Kind Regards,


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The trick is - There is nothing to worry about.

The ISP will send all traffic destined for your allocated addresses to you, all you have to do is deal with it when it arrives.

So:- the address allocated to the interface does not matter.


The convention is:-

Use one address for your general outbound originated internet traffic (PAT) and put that on your outside interface. Do what you want with the others.

If you are NATting everything then you just need to set up the NAT statements.


If you need further assistance please post the config with ip addresses sanitised and usernames/passwords removed along with your request. The best way to sanitise the addresses is to replace the first two octets say with x.y. This is likely to preserve useful information while still rendering you anonymous.

By the way, reading between the lines, the ISP is not using DHCP but, IPCP which (I suppose) is part of PPP.

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