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I'm considering the possibility of moving my public addresses to IPv6 for several web applications and sites that I host.

When inquiring with my ISP, the account exec replied to me with the following warning:

"I was checking in with the Engineer that is working on IPv6’s and he just sent me the following: Do they realize that if we give them IPv6 addresses they are, generally, only reachable from other IPv6 hosts?"

I'm not sure if I understand what is being referred to here. My public DNS provider has verified that their DNS services are fully v6 compliant and as long as my own equipment is capable, why would the ISP issue such a warning to me?

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They are warning you because IPv4 and IPv6 are two different protocols that don't talk to each other. If you run your server on only IPv6 then it will only be reachable by users that also have IPv6. Unfortunately that is still the minority (unless you target Belgian users, almost 50% off the users there has IPv6). And as good as all users with IPv6 also still have IPv4. The problem is that because of the shortage of IPv4 addresses the ISPs can't give everybody their own IPv4 address anymore. This lowers the quality of their IPv4 connectivity.

To make your server reachable in the most optimal way by everybody you need to provide both IPv4 and IPv6 side by side. That way users without IPv6 can still reach you over IPv4, and users that do have IPv6 don't need to go through the IPv4 mess. That way everybody wins, and the more services and websites support IPv6 the quicker the whole internet becomes independent of IPv4 and we can move on to an IPv6-only internet. But for now we still need both.

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    So to be backwards compatible in this case I'll need to run v4 and v6 simultaneously. Thanks for your answer! – Ted Quanstrom May 16 '16 at 14:45
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He's referring to the two being different protocols. They share the letters "I" and "P", but are otherwise as alien to each other as IPX and Appletalk.

In general, and IPv4 only host -- which is most of the world -- can only talk to other IPv4 hosts. The address fields are simply not large enough to accommodate v6 hosts. Also, most people don't bother with any sort of protocol bridging ("proxy") to get v4 into v6. (Just use v6, already)

IPv6, on the other hand, can hold many "internets". And seeing as very little of the world exists via IPv6, many people go to great lengths to allow their IPv6 infrastructure to reach the IPv4 internet. (see also: NAT64) For example, some cellphone networks are IPv6-only, but the provider runs a proxy to make IPv4 accessible -- and you don't even know it's happening.

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