Network & QoS
Double NAT
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
Double NAT is when your traffic passes through two routers that both perform network address translation. Typically that's an internet provider's combined modem-router feeding a second router of your own. It is a common cause of one-way audio, and of phones that keep dropping their connection to the phone service.
How it works
A router translates the private addresses inside your office into the single public address the internet sees. That is normal and harmless. Double NAT happens when it occurs twice: your provider's box translates, then your own router translates again behind it, so your phones sit two layers deep.
Voice suffers because a call's audio arrives as an inbound stream that has to find its way back to a specific device. Each translation layer has to remember which internal device a conversation belongs to, and when two layers disagree — or when one forgets sooner than the other — the return audio has nowhere to go. Hence the classic symptom: you can hear them, they cannot hear you.
Why it matters
It is common in small offices and almost always accidental. Someone keeps the provider's router and adds a better one behind it for Wi-Fi, and nobody realises the stack now has two translation layers. Calls then work most of the time and fail in ways that look random, which sends people chasing their phone provider for a problem that is inside the building.
The good news is that it is free to fix. Putting the provider's box into bridge or passthrough mode leaves exactly one router doing translation, and the symptoms usually disappear immediately.
Check the WAN address on your own router. If it is a private range — 10.x.x.x, 172.16–31.x.x, or 192.168.x.x — rather than a public address, something upstream is already translating and you are behind double NAT.
Related terms
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How do I know if I have double NAT?
Open your router's status page and look at its WAN or internet address. A public address means one layer of translation and you are fine. A private one — starting 10., 172.16–31., or 192.168. — means something upstream is translating too, and you are behind double NAT.
How do I fix double NAT?
Put the provider's modem-router into bridge mode, sometimes called passthrough or IP passthrough, so it stops routing and hands the public address to your router. Your provider's support can usually do it in minutes. The alternative is running only their router, but most offices keep their own for better Wi-Fi and control.
Does double NAT always break VoIP?
No, which is exactly why it is a nuisance to diagnose. Modern phone services handle it much of the time. It shows up as intermittent trouble — occasional one-way audio, phones dropping registration overnight, calls that fail only after a period of inactivity — the kind of pattern that gets blamed on the provider.
Is double NAT the same problem as SIP ALG?
Different faults with overlapping symptoms, and both live in the router. Double NAT is two layers of address translation confusing the return path; SIP ALG is a router rewriting call signaling badly. If calls misbehave, check both — disable SIP ALG and confirm you are not behind double NAT before escalating to your phone provider.