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How much overhead does a double NAT add to your outgoing packets?


G+_Ben Yanke
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How much overhead does a double NAT add to your outgoing packets? And more importantly, will this overhead affect me much, considering my slow connections? My two modems are actually Frontier internet all-in-one router/modem things, and they currently have NAT on, so they both have DCHP servers running which hand out an address to the main router. Past that, all the clients on the network connect to the main router, which dynamically routes connections toward each modem, depending on load.

 

Each modem has 1.5/0.3 mbps connections, so I don't have much bandwidth.

 

Modem 1 --> Main Router

Modem 2 --> Main Router

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I dunno if i'd call it overhead, id call it latency. Packets on the wan side of the router are totally unaware of the NAT. When the packets hit the router, it routes them to their translated addresses. For you to experience latency, you'd have to overload the router. With a wan connection like that, you are most certainly not going to notice. Either way, double NATs are generally bad news. Use vlans.

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I don't think double NAT would be noticeable under most scenarios - especially if there is only one device connected to the public-facing router. NAT works by the router changing the packet on its way out to mark itself as the sender. Each communication has an sender Address/Port combination and a destination Address/Port combo. The router creates a mapping of the sender port on the public side to your devices address/port on the inside. The increase in latency is from the router changing each packet with the updated Address and Port.

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Yup, Taylor! It uses both connections concurrently it isn't exactly bonded, as it is two discreet modems going out. As I understand it, the load balancing router routes roughly half of the TCP/IP connections out each modem. This means that when downloading a large single file (ie, one connection), you only can get the bandwidth of one modem, but if you are browsing or have multiple users using the LAN concurrently (lots of connections), you effecticely get the combined bandwidth of the modems.

 

I don't know the model of the router, but I really like it, and I'll post the model number when I get home.

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That's a nice peice of kit you have there. If I had more than 1 ISP available where I live, I'd definitely consider such a setup. Not going to give the same ISP twice the money though, as when the ISP goes down, both links would die.

 

It would seem that double-NAT isn't a major issue for your setup, but you'd probably be better off putting the modems in bridge mode, shutting down all but what's necessary for the modems to run their DSL links, and let that nice router you have run your network.

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I've considered 4G for a customer who can only get 1.5Mb with his DSL provider, and the cable tv provider wants an inordinate amount to bring coax down his street. I'm leaning toward a point-to-point link to a neighbor with sufficient bandwidth. I already have some parabolic grid wifi antennae laying around from previous projects, as well.

 

For a while, I was getting torrents ONLY over the neighbornets :)

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Nice. Stealing neighbor's wifi for your torrents. I've thought of this same solution though for people who can't get better than DSL. Seems like if you could get cable 5 miles away then you could use Ubiquiti point to point access point - that's a lot of points - links to get the faster service.

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Ignorance is not an excuse, and an open network is an invite for free access. Now that we have a home in the countryside AND adequate bandwidth, I don't piggy-back on neighbornets. I actually helped a few secure theirs, only later to hear from other neighbors they aren't getting the free wifi anymore :)

 

But, yeah, AirMax or similar would be great kit to have. You can never have too many points :)

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