Assigning internal address ranges

[Theojt]

Re: RT-N56U with firmware version 3.0.0.3.165

This is kind of a network design question from a bit of a noob.

My ISP’s DSL “modem” is configured for DHCP on the WAN side and as 192.168.1.1 on the LAN side.

I connect the WAN side of my RT-N56U to the LAN side of the modem with “Automatic IP” and a fixed address of 192.168.1.2 (subnet mask 255.255.255.0) on the LAN side.

When I did this, the firmware reconfigured itself, explaining that I needed a different subnet for my internal LAN – which it made 192.168.2.x

So – is this a best practice? Does it really matter? My prior software version did not enforce a different subnet and seemed to work fine with a 192.168.1.2 address (v1.0.1.8i).

And this seemed weird – clients that I changed to point the gateway at 192.168.2.2 got a 192.168.2.x address. However, clients that still pointed to the old gateway address at 192.168.1.2 received 192.168.1.x addresses. This looks like the DHCP server provides addresses in either range, which doesn’t sound right.

Can someone please explain or point me to a reference where I can understand what’s happening in more/better detail?

 

[RMerlin]

Best practice would be to see if your modem has an option to be put into bridged mode and disable all its router functionality. Once done, your DSL login credentials (PPPoE or otherwise) will be entered in the router instead.

Otherwise, with your current setup you will be doing what is known as "double NAT", which will cause various issues, most visibly the fact that you won't be able to do port forwarding on the router unless you also forwarded those same ports on your modem.

 

[thiggins]

You can't have two routers operating in the same subnet.

If your DSL modem/router is using 192.168.1.X, then a router attached to it must use another private-range subnet. As Merlin points out, this results in a double-NAT situation.

I have run double-NAT for years, but don't forward and ports or run any services that need WAN-side access. The services I do use: Skype, Pogoplug, WD2go, etc. work just fine through multiple NAT.

 

[Theojt]

Thanks. My modem is bridged - maybe I just need to turn off DHCP on it. There is no logon that I am aware of for my DSL service.

Jeff