Connecting 7800DXL to another router with a different subnet

[Moongit]

Hello. I wonder if anyone could help me out here. I have a 7800DXL as my main router, it's a great bit of kit, and I connect to it wirelessly and with wires from my devices. Up until now, my Internet connection (BT Fibre to the curb, AKA BT Infinity) has been supplied through a standalone modem. Essentially I had an Ethernet cable connected to the modem and that goes into the 7800DXL's WAN port, and the 7800DXL is setup to connect via PPPoE. My router is obviously setup to handle DHCP. The router's IP address is set to 10.20.1.1 with the DHCP server having the range 10.20.1.2 - 10.20.1.254.

It all worked flawlessly until my modem started playing up, and after talking to my ISP they sent me a new one. Except, they refused to send me a like for like replacement and gave me this "HomeHub5", which as you may or may not know, is a fully fledge router just like the 7800DXL and cannot be switched to "modem" only mode. Now, my problem:

I cannot change the 7800DXL's IP, or DHCP range. For reasons I won't go into the devices on the network must have IPs beginning with "10.x.x.x" and this is why I have an issue, all I'll say is it's job related but I'm under NDA so I can't explain why I can't change the IPs. The problem arises because the HomeHub5 refuses to allow the use of the 10.x.x.x IP addresses for the LAN as it is reserved. The HH5 you see is designed also to act as a wireless hotspot for anyone with a BT Broadband account and they use that IP range for that specific purpose and of course, lock it away from being used by the home user.

Normally, what I'd do to get the two routers to play nice with each other is simply change the IP address on the HH5 to the same subnet as the 7800DXL, disable DHCP on it and any other features I don't want, and then stick a static IP on the 7800DXL for the HH5. I've done this before with a 7600N and an old 3COM router, when the 7600N's WiFI conked out, and it worked well enough. Of course because I can't change the Ip of the HH5 to be the same the 7800DXL I'm stuck as to what to do.

Would a static route work? That's the only thing in the 7800DXL's settings that even vaguely seems relevant to my issue but I don't know how to set it up if it is. Ultimately, all I want the HomeHub5 to do is connect to the Internet as a modem and feed the connection through ethernet to the 7800DXL, however without needing to change any IP addresses which is the main problem of course. Can anyone help me out?

[billion_fan]

Hello. I wonder if anyone could help me out here. I have a 7800DXL as my main router, it's a great bit of kit, and I connect to it wirelessly and with wires from my devices. Up until now, my Internet connection (BT Fibre to the curb, AKA BT Infinity) has been supplied through a standalone modem. Essentially I had an Ethernet cable connected to the modem and that goes into the 7800DXL's WAN port, and the 7800DXL is setup to connect via PPPoE. My router is obviously setup to handle DHCP. The router's IP address is set to 10.20.1.1 with the DHCP server having the range 10.20.1.2 - 10.20.1.254.

It all worked flawlessly until my modem started playing up, and after talking to my ISP they sent me a new one. Except, they refused to send me a like for like replacement and gave me this "HomeHub5", which as you may or may not know, is a fully fledge router just like the 7800DXL and cannot be switched to "modem" only mode. Now, my problem:

I cannot change the 7800DXL's IP, or DHCP range. For reasons I won't go into the devices on the network must have IPs beginning with "10.x.x.x" and this is why I have an issue, all I'll say is it's job related but I'm under NDA so I can't explain why I can't change the IPs. The problem arises because the HomeHub5 refuses to allow the use of the 10.x.x.x IP addresses for the LAN as it is reserved. The HH5 you see is designed also to act as a wireless hotspot for anyone with a BT Broadband account and they use that IP range for that specific purpose and of course, lock it away from being used by the home user.

Normally, what I'd do to get the two routers to play nice with each other is simply change the IP address on the HH5 to the same subnet as the 7800DXL, disable DHCP on it and any other features I don't want, and then stick a static IP on the 7800DXL for the HH5. I've done this before with a 7600N and an old 3COM router, when the 7600N's WiFI conked out, and it worked well enough. Of course because I can't change the Ip of the HH5 to be the same the 7800DXL I'm stuck as to what to do.

Would a static route work? That's the only thing in the 7800DXL's settings that even vaguely seems relevant to my issue but I don't know how to set it up if it is. Ultimately, all I want the HomeHub5 to do is connect to the Internet as a modem and feed the connection through ethernet to the 7800DXL, however without needing to change any IP addresses which is the main problem of course. Can anyone help me out?

Easiest option is to get a open reach VDSL modem off fleebay, problem solved. (so you can avoid the double nat issues)

[Moongit]

That is certainly the easiest option but I'd still like to know if it can be done the way I'm asking, since that would save me having to pay for a modem BT used to give away for free.

[billion_fan]

That is certainly the easiest option but I'd still like to know if it can be done the way I'm asking, since that would save me having to pay for a modem BT used to give away for free.

Well some others have just configured the 7800DXL for a Ethernet connection (EWAN) >> IPOE (IP Over Ethernet) the 7800DXL's WAN will obtain a internal IP address from the BT Home Hub 5, eg 192.168.1.x, then login to the BT hub 5 and DMZ the IP address the 7800DXL's WAN has obtained, this way the 7800DXL can still handle the DHCP requests etc. (there is no way to get the 7800DXL to obtain a WAN IP, because of the limitation on the BT home Hub)

[Moongit]

Thank you. I may try it that way, then again I'm not sure I can be bothered with the hassle! For the time being I'll just use this spare modem I've got until such a time I can fix the old modem or replace it. This home Hub 5 can rot in the box!