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What kind of Nat though? You can use upnp, predictable mapping, etc. and still allow the traffic through. And that's only with ipv4, because you can run zerotier over IPv6.



> What kind of Nat though? You can use upnp, predictable mapping, etc. and still allow the traffic through.

Your computer can talk to your home router (CPE) and punch a hole for a connection, but if your WAN port does not have a public IP address, but rather itself also has a private address (probably 100.64/10), the CPE cannot talk to the ISP's router to punch a hole:

* https://en.wikipedia.org/wiki/Carrier-grade_NAT

The two layers of NAT (home network (192.168) -> CPE NAT (100.64/10) -> ISP NAT ('real' public IPv4)) prevent hole punching.


Double Nat on one side is not that universal. Across Europe and Australia I've seen it maybe once on a residential connection. I'm sure it's used, but the comment about the US in the post above just doesn't match my experience.


Great for you for not having to experience it, but that doesn't mean it sucks any less for those less fortunate:

> Our [Native American] tribal network started out IPv6, but soon learned we had to somehow support IPv4 only traffic. It took almost 11 months in order to get a small amount of IPv4 addresses allocated for this use. In fact there were only enough addresses to cover maybe 1% of population. So we were forced to create a very expensive proxy/translation server in order to support this traffic.

> We learned a very expensive lesson. 71% of the IPv4 traffic we were supporting was from ROKU devices. 9% coming from DishNetwork & DirectTV satellite tuners, 11% from HomeSecurity cameras and systems, and remaining 9% we replaced extremely outdated Point of Sale(POS) equipment. So we cut ROKU some slack three years ago by spending a little over $300k just to support their devices.

* https://community.roku.com/t5/Features-settings-updates/It-s...

* Discussion: https://news.ycombinator.com/item?id=35047624


You can't over double NAT because the second layer of NAT is not going to support UPnP


The vast majority of CGNATs across the world don't support the PCP protocol for predictable mapping.




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