Splitting DHCP from ADSL modem?

**Scuba_Steve** · 7th July 2014, 13:02

Hopefully someone here will know the in's & out's to be able to offload the DHCP from the ADSL modem to a router?

What I have is an ADSL modem which I still want to do the PPPoA auth but I don't want it to distribute the ext IP to the many internal ones via DHCP I want that job to be passed onto the router attached to it.
The router only supports PPPoE auth so I can't pass that ability over & I'm trying to avoid double NATing if that is at all possible.

So how do I setup the ADSL modem to do the PPPoA authentication but for the router to issue the DHCP & effectively everything else?

**Big Dog** · 7th July 2014, 13:07

Set a static on the modem and routers. Enable dhcp on the router. Job done.

Stupid phone / Tapatalk, apologies in advance.

**Big Dog** · 7th July 2014, 13:09

Ps you'll. need to either reserve the 01 IP address in the dhcp for your modem or set your newly chosen static ip as your default gateway.

Stupid phone / Tapatalk, apologies in advance.

**Scuba_Steve** · 7th July 2014, 13:12

So you're saying something like

ADSL=192.168.1.1; DHCP=Disabled
Router=192.168.1.2; Gateway=192.168.1.1; DHCP=192.168.1.5 - 192.168.1.254

**Big Dog** · 7th July 2014, 13:13

I would also recommend setting an alternate up address on at least one PC. In case you ever lose dhcp off the router. Such as you need to swap it out with another router. Otherwise first hint you have an issue is none of your devices can connect once their registrations expire.

Stupid phone / Tapatalk, apologies in advance.

**Big Dog** · 7th July 2014, 13:14

Originally Posted by Scuba_Steve

So you're saying something like

ADSL=192.168.1.1; DHCP=Disabled
Router=192.168.1.2; Gateway=192.168.1.1; DHCP=192.168.1.5 - 192.168.1.254

That was pretty much how mine was setup when I had a separate router for wireless.
I put registrations in for each network devices MAC address

Stupid phone / Tapatalk, apologies in advance.

**Big Dog** · 7th July 2014, 13:16

Your ADSL will need two addresses.
One internal. 192.168.1.1 and one public ( you get this dynamically from ISP unless you are I a static ip)

Stupid phone / Tapatalk, apologies in advance.

**Gremlin** · 7th July 2014, 13:27

Looks like you'll get it sorted.

You can only have one DHCP server within a network. Basically, turn off DHCP in modem, turn on in Router. Set appropriate range for DHCP (and make sure nothing has a static address inside the range).

**Big Dog** · 7th July 2014, 19:07

Originally Posted by Gremlin

Looks like you'll get it sorted.

You can only have one DHCP server within a network. Basically, turn off DHCP in modem, turn on in Router. Set appropriate range for DHCP (and make sure nothing has a static address inside the range).

If you desire a static IP within the range that is where reservations come in.
You can still apply the static IP but if not reserved that address may be handed out to another competing device.
If you use an IP high enough in the range so as to not ever be a cause of conflict (eg using .36 when you know there are only 25 devices) then you will be fine... but if you are going to have lots of static ip's (printers, NAS etc) better off having a separate range if your router does not support reservations.

**Scuba_Steve** · 7th July 2014, 19:51

part the reason I wanna use the router rather than the modem is the router supports IP reservations & a guest network whereas the modem doesn't
I'm gonna give the setup ago tomorrow & hopefully it all runs smooth as the wife doesn't like to be disconnected from the interwebs for long too used to having it & I don't want the beatdown

**Gremlin** · 7th July 2014, 21:12

Originally Posted by Big Dog

If you use an IP high enough in the range so as to not ever be a cause of conflict (eg using .36 when you know there are only 25 devices) then you will be fine... but if you are going to have lots of static ip's (printers, NAS etc) better off having a separate range if your router does not support reservations.

Addresses aren't always handed out in order, can be random. RouterOS hands out top down more or less actually...

**Big Dog** · 7th July 2014, 21:22

Originally Posted by Gremlin

Addresses aren't always handed out in order, can be random. RouterOS hands out top down more or less actually...

All the domestic hardware I have used or set up for others starts at the bottom and scales up, releasing unused IPs after 14 days. The commercial gear I have used scales down, unless you set rules to the contrary.

Stupid phone / Tapatalk, apologies in advance.

**Big Dog** · 7th July 2014, 21:25

Sure makes guessing the IPs of printers etc easier.

Fwiw printers are shocking on DHCP without reservations. They end up out of synch with clients without a print server. Even then some brands are useless at staying in synch.

Stupid phone / Tapatalk, apologies in advance.

**Gremlin** · 7th July 2014, 23:00

Who puts a printer on DHCP (esp without reservation)?

One down on my list from some gormless tech that puts a static address on a device and the address is inside the DHCP range... Can take you months of random issues before you finally track down what's going on... (or you drill holes throughout your DHCP range)

**Big Dog** · 8th July 2014, 00:20

Originally Posted by Gremlin

Who puts a printer on DHCP (esp without reservation)?

One down on my list from some gormless tech that puts a static address on a device and the address is inside the DHCP range... Can take you months of random issues before you finally track down what's going on... (or you drill holes throughout your DHCP range)

There are times on a home network the simplest solution is the best solution.

Stupid phone / Tapatalk, apologies in advance.