DHCP Server in Modem or Router?

captainpat

I have ADSL from UTV. BT Voyager 210 modem, connected to Dlink DI524 4-port wireless router. Modem IP is 192.168.1.1, router IP 192.168.0.1. I have been getting messages about problems with NAT since I joined with UTV. When investigating settings on modem and router, I saw that the DHCP Server service was enabled on both units. This did not seem right, so I turned off the service on the Dlink router. I restarted everything, but now I had no internet connectivity. I did a release/renew on one machine, but did not get an IP. So I re-enabled DHCP on the router and restarted OK.

I am preparing to turn off DHCP service on the modem, and want to make sure I don't kill everything. I should be able to connect a PC direct to the modem if I need to turn DHCP back on.

Am I on the right track? As far as I know, the eircom ADSL router is similar to the UTV BT one, so what I have must be a very normal upgrade. Could anyone with a similar setup advise if both DHCP Servers can be on, and if not, which one should it be? Also, will I need to do any Port Forwarding between the units in order to support the configuration? Or any other advice would be very welcome. Thank you.

macrubicon

The ADSL modem is in essence a Router. So what you have is a Router connected to a Router.

Not usually a problem if the "outside" of the DLink is set to take it's settings from the UTV "Inside"...

I.e. DHCP on the UTV router gives the DLink an address. The Dlink then gives out DHCP leases on a different scope to your LAN.

The issue arrises when traffic is moving between the two.

The DLink will NAT everything out the 1 IP it got from the BT modem. So you are translating from 192.168.1.0/24 to a single IP on 192.168.0.X.

If the traffic is bound for the outside, it will again be NAT'd by the BT modem from the 192.168.0.X address to whatever your external IP is.

In effect you are NAT'ing twice. This can cause problems as things can get very confused as to what goes where and when.

What I would do in these type of cases is try to use PPPoE on the "inside" device - in your case the DLink. I'm not sure of the BT stuff, but with the Eircom netopia devices you can set them to Bridge mode ( I think that's what they still call it ! ) when basically means it hands off all services to the DLink. The modem's LAN in essence acts like a hub instead of a switch.

The DLink basically becomes the one active device and you end up with 1 DHCP server and scope and 1 NAT operation directly at the egress of your network. Provided the DLink supports PPPoE on one if it's interfaces.

Sorry for the long answer and I hope it helped.

captainpat

Will Set it up as you suggest when I get home.