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Jason
Asked: 2015-06-24 19:41:21 +0800 CST

Is there any disadvantage to putting a modem in bridge mode

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In order to prevent the need of port forwarding and double natting I plan to put a modem into bridge mode. Are there any side effects or problems that could come of placing a modem into bridge mode in order to allow for easier access to internal services?

networking

2 Answers

  1. Best Answer

    Ideally (ie pedantically) speaking, your modem is always be in bridge mode, but many "modems" also have a router built in to handle NAT etc for you. It's also common enough to have that handled in a separate device, leaving the modem to do what it does best.

    As you've mentioned double natting, I'm assuming you have a dedicated router. I'm also going to assume that your router can speak PPPoE (or whatever protocol your ISP uses). One thing you'll need to check is that your ISP has not locked the connection down to only talk to the MAC on your modem's internal switch. If this is the case, you can either have them add your router's MAC to their access control list, or to change the MAC on your router's external interface. Neither approach needs to be an overly daunting prospect.

    Aside from those wrinkles to consider, there's absolutely no issue with bridging on your modem. You'll be presenting the assigned IP from your ISP directly to your router's external interface. It does mean that you need to pay attention to your router's security configuration and to remember that your ISP supplied modem is going to let everything through now.

    Other options / points to consider:

    • If bridging doesn't work out and if your modem has a poor man's "DMZ mode" that forwards all ports to a given internal address, use it to set your router as being in a DMZ of sorts, then have it route on to your "private network".

    • If your modem insists on handling the PPPoE and just bridges the IP connection over the top, you can still have the router handle the NAT for you.

    • Think about your link speeds and MTU settings; make sure your router is sending packets sized appropriately for the modem and that it doesn't send too much traffic for the modem's (probably small and crappy) buffer, do your buffering back on the router where it can make the most sensible decisions around QoS etc.

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  2. Are there any side effects or problems that could come of placing a modem into bridge mode in order to allow for easier access to internal services?

    Well, that's probably something you should talk with your ISP about.

    In general, though, it's a great idea. Double-NAT is evil.

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