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I'm wondering what benefits there is to using "bridged" mode on a modem. If your router can handle PPPoE, is there any reason to bridge it over to the router, rather than having it run on the modem?

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Whichever device does PPPoE also has to do NAT, and whichever device does NAT usually does DHCP. If your router has a better, more flexible NAT and a better DHCP server, let your router do it.

You don't want both devices doing NAT if you can avoid it. That's called "double NAT" and tends to cause problems for people who don't know what they're doing (and even some who do).

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+1 It also makes the modem a 'dumb' bridge, which may help with administration. Now all config is in the router, giving a single point to investigate on failure. – Rich Homolka May 17 '11 at 23:17
Well, I can configure my modem so that the DHCP assigned address is a public IP address.. So if I configure it to do that, preventing double-NAT, that is "basically" no benefit to running PPPoE on the router? – Earlz May 18 '11 at 0:33
Can't you do double-NAT? That is on the modem you route everything to your router, and then you configure your router normally. There is also no reason to run DHCP on the modem at all then. Is this wrong? – zespri Mar 20 at 23:48
@RichHomolka, well modem is still a piece of hardware and it still can fail. It's just that if it's in pass through or bridge mode it's harder to diagnose now - so you gain some, you lose some. – zespri Mar 20 at 23:49

Spiff is correct...


If the DSL modem handles the PPPoE connection, then the modem will distribute private, non-public internet IP addresses to the device / devices connected to its ethernet port with incoming traffic from the internet. The process of moving between public (internet) and private (internal network) IP addresses is called Network Address Translation (NAT). NAT can also be considered a firewall of sorts because it masks the internal IP's / individual computers.

Depending on how many IP addresses the DHCP server on the modem handles, you could technically connect a switch to the ethernet port and the modem will hand out private IP addresses to the devices that are connected and you wouldn't need a router.

If you would like to have a router to provide more control like a better firewall, wireless, and possibly more devices connected, then you would opt for a wired / wireless router.

By default, a router would acquire an IP address from the modem (private or public) and would perform NAT and isolate the devices from the "outside network". If the modem is handling the PPPoE, then the router would receive a private IP from the modem and then create another private network via NAT. There would be two layers of private networks in this instance and is recognized by many devices as "Double NAT" which may cause issues with traffic trying to find its way in and out of your network and possibly increase latency.

There are two ways you can mitigate this issue:

  1. Configure your router to not hand out DHCP addresses and pass the IP addresses handed from the modem to the clients on the network. You must keep in mind that many modems have a set amount of devices that can receive IP addresses from its internal DHCP server.

  2. Configure your router to handle PPPoE. The router would then have the public IP address assigned to the WAN port and would have one layer of NAT between the internet and the client computers on your network.

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I normally keep running PPPoE on the modem to ensure a tiny bit lesser load on my router and let the modem do the job it was specifically optimized to do best. However, I sometimes switch it to the modem for two reasons:

  1. The disconnect logs (for troubleshooting) on the modem tend to be nonexistent on some modems, hidden behind needless clicks, and/or require a login password that you're guaranteed to forget lots more than your router's.
  2. Many DLINK routers have a globe-shaped "internet" LED that goes orange when PPPoE is down.

In my case the modem's lights are hiding or set off to one side. Even when they're in front of me, it is harder to know that PPP is temporarily down because the modem just turns off one green indicator in a row of 4 or 5 other equal green indicators.

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