Avoid private IP overlap

julianov403 · ‎09-03-2017

Hello All

I've a ADSL modem which it's 3 layer. The ISP gives me only one ip public, So the modem is the equipment which implements the nat. Well, I have an IP PBX, an Asteriks running on a physical server but as I want to implement a QoS and the modem do not has this implementation. I thought that I can use a router for that, a Cisco 2811

The LAN gateway would be the 192.168.0.1 (router's port) and the router will route the trafic throught 10.0.0.0/30. But my modem implements a DHCP with 10.0.0.0/24 as ip pool. Other implements modems uses 192.168.0.0/16 or 192.168.0.0/24 ip pools for the DHCP server, so there would be overlap and I want a solution that can be make in every situation. So, how can I do in a way that there wouldn't be ip overlap. You can says that I must changes the modem's ip but for example, I can't changes the ip pools in my modem.

I was thinking that with MPLS I can fix it.

Georg Pauwen · ‎09-03-2017

Hello,

not sure if I fully understand what you are asking, but:

1. If you set the router interface connecting to the modem to DHCP, it will get an IP address form the 10.0.0.0/24 address space.

2. The router can serve as a DHCP server, and you can configure it to dish out IP addresses in the 192.168.0.0/24 range.

What do you mean by avoiding overlap ? In the above scenario, there is no overlap. That said, you need to make sure that your modem does NAT also for the 192.168.0.0/24 space.

julianov403 · ‎09-03-2017

Thanks for the quickly response.

Ok lets see.

@Georg Pauwen wrote:
Hello,
not sure if I fully understand what you are asking, but:
1. If you set the router interface connecting to the modem to DHCP, it will get an IP address form the 10.0.0.0/24 address space.
2. The router can serve as a DHCP server, and you can configure it to dish out IP addresses in the 192.168.0.0/24 range.
What do you mean by avoiding overlap ? In the above scenario, there is no overlap. That said, you need to make sure that your modem does NAT also for the 192.168.0.0/24 space.

The problem is when you can't make sure that the modem doesn't implements 192.168.0.0/24 space-

First off all I will configure a DHCP server on the rotuer with the 192.168.0.0/24 as ip pools. This LAN is connected to Gi0/0 (for example). And in Gi0/1 there is the connection between the modem and the router. The Gi0/1 ip address is given by the modem DHCP server.

If you ip pools used by the modem is 10.0.0.0/24 there is no problem but if the modem use 192.168.0.0/24 there would be an ip overlap. The problem here is that I can't control or configure de modem's ip pools. So there is a solution to avoid it when the router dhcp and the modem dhcp servers uses the same ip pools? For example configure virtual interface and use MPLS

Now I'm thinking that to avoid it I can configure the dhcp with the carrier grand NAT ips in a way that it would not overlap any IP private.

As you can see it don't care what public IP there is in the network between the routers (or between router an a modem). The LAN will use 10.64.0.0/16 and as there is a nat in the first router there wouldnt be a problem with any filters.

But maybe there is another solution that use the carrier grade NAT IPs

Georg Pauwen · ‎09-03-2017

Hello,

the first thing to find out is what the modem DHCP server actually dishes out. What modem (brand/type) is that actually ?

That said, on the LAN side, are you forced to use 192.168.0.0/24 (are there devices already configured with these addresses, and you cannot change those) ?

julianov403 · ‎09-03-2017

I've to do a fixed solution which must run in every posible situation. Always there will be a modem but the modem can has differents ip pools for their dhcp server and I cant go and configure the modem just send the router which is alredy configured. So, In one of that situation there can be ip overlap and I've to implement something that fix that problem. For example configure the router in a way that first "sense" the modem dhcp server and then use a different ip pool to avoid the overlap, that would be something like an script, or maybe use MPLS or the 100.64.0.0/10 space.

Georg Pauwen · ‎09-03-2017

Hello,

what type/brand is the modem ? These modems are typically configured for home use and dish out IP addresses in just one range. Connect the router and find out which network that is...