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[kitzcurly]

I received my new router super-quick nd have set it up as modem, replacing my HG612.

Question is, should I disable DHCP on the modem? I can see on the Connection Status screen that it has two clients which seems weird to me, and DHCP is enabled as per default.

When I try to disable DHCP I get the error "This IP Address is used by another Group."

When I try to switch to DHCP Relay and point to my router IP address I get the error "Please disable NAT if you want to enable DHCP relay". I have hunted through all the settings and disabled anything referring to NAT.

[johnson]

How are you connecting the modem to your router? A single cable? have you set up interface groups as per:
https://kitz.co.uk/routers/zyxel_VMG8324-B10A_bridge.htm

I'm not entirely sure how you have access to the modems GUI if you are using a single cable and have not changed anything on your router to allow access. Unless you are using a laptop or something to connect directly to the modem? You may find once you have interface groups set up you lose access to the modem without a second cable from the router.

Lots of assumptions there, tell us a little more about have you have it set up. 

[Weaver]

If you want to use your router as just a straight modem, ie turning it into just a modem, that means connecting it to your router and getting the router to speak PPPoEoE to it. People sometimes refer to it as ‘bridged mode’. I just want to confirm that that’s what we’re talking about.

This is what I do with my modems. I have ZyXEL modem-routers and have them configured to be in straight modem-only mode.

Now in this case, dhcp server function on the device doesn’t mean anything, as it isn’t bring a router any more and acting as a dhcp server to PCs etc on the LAN, handing out in addresses etc, is not something that it will be doing any more. Being a dhcp server for PCs etc is now a job for your main, real router (or some other dhcp server) on your main lan.

If ‘dhcp’ is still enabled on the now-modem then if that means dhcp server then it won’t be doing anything.

If (really unlikely) it means ‘be a dhcp client’, then meaning ‘get myself an IP address from some dhcp server somewhere’ then that might or might not be quite useful, as we need to decide somehow on an IP address for the admin function of the LAN i/f of the modem if we want to talk to it, use its web UI, get stats from it or whatever. We might have to use two Ethernet cables, one to get stats and do admin, and one for the user data connection to the router via PPPoEoE, if the router is going to prevent your talking through it to get to the modem.

Just to get data in and out of the modem to and from the internet via the router you do not need an IP address for the modem, because pppoe doesn’t use IP at all only lower level Ethernet stuff.

However, as I said, if you want to administer it, you will have to decide on an IP address for the admin interface, if not, then it doesn’t matter. There has been a lot of discussion about getting admin and stats access set up in other threads and articles here.

I had a lot of brain ache with my router working out how to get access to the modem _through_ it, as I wasn’t sufficiently familiar with the advanced features it has and I also chose ip addresses for the modem’s admin function which made things awkward for router setup (and for modem setup too). But I have four modems so life was likely to be hard.

[kitzcurly]

Thanks for the replies.

Yep I failed to explain that my Zyxel is setup using the explanation here:https://kitz.co.uk/routers/zyxel_VMG8324-B10A_bridge.htm.

The Zyxel is replacing a HG612 and an Asus RT-AC68U is on the other side of it in router mode with DHCP enabled. I followed the instructions to set up the 2nd interface group and have a 2nd ethernet connection to a LAN port on the router using an IP address I assigned in the same 192.168.0 range as my router and its DHCP clients.

It makes sense that in this setup the modem shouldn't be giving out IP addresses and that's why I tried to switch it off when I saw the DHCP server is on on the modem. I'm in the Home Networking menu. Please see screenshot attached.

[johnson]

Great, thanks for the info.

If you scroll up a little and check the "Group Name :" drop down box, try selecting the other one and fiddling with the DHCP settings on that? Also check the IP it has been assigned, it does it automatically usually to 192.168.2.1.

You do want DHCP to be disabled on the LAN interface from the modem as it will be a race for which device responds soonest to a DHCP request from clients on the lan if its still enabled.

I have experienced some weirdness with these Zyxels when choosing a different IP for each of the "Groups" after setting them up. I struggle to recall but it may have required setting the default IP to something else before (or maybe after?) creating the groups. 

[kitzcurly]

Ah, there you go.

Yes if I select Bridge from the drop-down it shows 192.168.2.1 and DHCP is disabled. I cannot connect to 192.168.2.1.

I tried changing the Bridge IP to 192.168.0.2 and it looked like it was going to work (I got the warning that the IP address was changing) but I can't connect to the new address 192.168.0.2 and when I connect to the LAN IP address of the modem (192.168.0.1) and again select Bridge the IP address hasn't changed - its still 192.168.2.1.

I guess it doesn't matter that the modem is using 192.168.2.1 but as you say it seems logical that I should be able to disable the DHCP server on the LAN interface. Otherwise I assume there's a risk of duplicate IP addresses and all sorts of fun and games.

[johnson]

There should be no need to change the "bridge" connections IP, and certainly not to the same exact IP or subnet as the LAN groups one. I just suggesting looking at it to see if it was conflicting.

You could try changing it to 192.168.3.1 or 192.168.4.1 or anything outside of the 192.168.0.1/24 subnet you use for your LAN.

Its a wild guess but given you said it gives the error "This IP Address is used by another Group." I would attribute it to some poorly coded checks in the Zyxel interface that expect the IP to be something else, so changing the others may have some effect. 

Edit:
In the screenshot you have the modems LAN IP set to 192.168.0.1. What is the Asus routers IP set to?

[kitzcurly]

I was indeed able to change the modem bridge IP to 192.168.3.1.

Still can't disable DHCP server on modem LAN side.

Router is 192.168.0.254.

[hushcoden]

I think you can't disable the DHCP server on ZyXEL LAN side if you're connected to it trough cable as the modem, i.e. its DHCP server, has assigned a second IP address to your PC Ethernet port... one IP address goes to the modem LAN port and a second one goes to your PC Ethernet port...

[kitzcurly]

I think you can't disable the DHCP server on ZyXEL LAN side if you're connected to it trough cable as the modem, i.e. its DHCP server, has assigned a second IP address to your PC Ethernet port... one IP address goes to the modem LAN port and a second one goes to your PC Ethernet port...

Not sure that's the problem - my various laptops are all getting their IP from my router (their IPs haven't changed since I configured the new modem), and furthermore I'm connecting to the LAN side of the modem through the router (modem has 2 cables to router, 1 for WAN side and 1 for LAN side as descibed in the bridging guide on this site).

[Weaver]

Since if I’ve understood correctly (apol in advance) the modem’s admin interface is now in the main LAN, it’s exposed to nefarious access by miscreants and evil apps. Your LAN may be such that there never are any such miscreants or guests; it may be physically secured. I would just say that it’s worth setting a non-default admin login on the modem just in case.

I myself have done so: random long admin and supervisor passwords. My own modems are not directly on the main LAN, but behind the router. They don’t use IPv4 addresses that are within my main LAN range. But my modems are still supposedly accessible, albeit through the router, as opposed to directly. So I have also firewalled the pathway to the modems so only certain client machines can access them at all.

I seem to remember some evil code that attacks routers and modems, so it’s best to be paranoid. Has there been a few bugs + security holes in the ZyXEL code? - relating to unauthorised access without login. Does anyone know?