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As the ISP-provided modem was generating pretty weak WiFi signal I've disabled its WiFi function, bought an Apple AirPort Extreme and connected it's WAN port to the modem. Now everything works just perfectly, but the AirPort configuration utility keeps complaining (yes, I know how to turn it off, but I am curious) about double NAT and demands me to reconfigure the modem to bridge mode. Why should I? What problems can double NAT cause?

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Added latency? probably not even a couple ms. Your setup is no different than many vitual machine setups. –  hydroparadise Dec 18 '12 at 14:18

2 Answers 2

up vote 4 down vote accepted

Anything that automagically open up holes in your firewall (for instance, as BitTorrent client might use uPNP to get a port opened for itself without direct user intervention) is going to fail, because it can't access the 'outer' NAT.

Otherwise, it's a bit of added latency (not likely to be significant) and you're paying to power two devices instead of one.

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You should be able to get around the uPNP problem by simply setting DMZ on the modem ("outer device") to point to the "inner" Apple Airport. –  Hengjie Apr 7 at 22:21
    
@Hengjie - at that point you're not doing double-NAT, you're doing single NAT with the inner airport in the DMZ. So really a different situation. Which would in fact solve the uPNP issue, but the OP was asking what issues arise from double-NAT, not alternate network topologies. –  Michael Kohne Apr 8 at 10:39
    
you would still be double NAT'ting because both router will run NAT. It's just that the outer router will DMZ pointed the outer router's DHCP negotiated address for the inner router. So you will continue to incur the cost of two network translations, but using DMZ simply fixes uPNP for the inner router. –  Hengjie Apr 9 at 10:58
    
To add to the accepted answer sometimes you need to double NAT if you have special hardware like VOIP router and you do not want to use it for all your network services. One configuration that I have home is fritz box with VOIP as modem with DMZ pointing at a time capsule which is the real network router and wifi poa. Both routers, if set in bridge mode, do not work properly (one refuses to service as VOIP box, the other as backup server), therefore I run double NAT. –  Daniele Bernardini Apr 22 at 14:03
    
I had some problems with a similar configuration years ago resulting in a lot of dropped packets and a very slow connection but otherwise I am running similar configuration in multiple offices and apartments since years with negligible latency impact and no problem whatsoever. –  Daniele Bernardini Apr 22 at 14:03

I had tried double and triple NAT-ing setups for fun. For most intents and purposes, double NAT-ing doesn't affect simple browsing/mail experiences. (adds less than 1ms of latency)

However, if you want remotely access your home network (or services) from internet. It would be a lot more complicated to set that up. That's the only disadvantage I can think of.

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