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Given that I have a dedicated public IPv4 address for a web server (Ubuntu).

What are the real disadvantages of disabling IPv6 for our users?

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    You won't be prepared for the Internet to go v6-only. You really should run a full implementation of v6 if your infrastructure supports it. Oct 27, 2017 at 3:16
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    You won't be reachable for IPv6-only clients.
    – blafasel
    Oct 27, 2017 at 8:51
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    In short: IP communication involves two parties. You'll need to consider not only if you have good IPv4 connectivity, but also whether all of your potential users do. The answer to that is usually "no, many of them don't" these days. Oct 27, 2017 at 11:35
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    +1 for asking rather than just assuming that turning off IPv6 is a good idea like too many other admins have done.
    – kasperd
    Oct 28, 2017 at 7:53

2 Answers 2

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By disabling IPv6 you - obviously - lock out any v6 traffic. Users behind NAT routers may be sharing the same IPv4 address - in case you want to be able to tell them apart.

This doesn't only apply to home / company users behind an on-premises firewall, it also applies to various other scenarios, e.g. Internet access providers that have run out of v4 addresses and put their customers behind private/public SNAT but also to Dual-Stack Lite customers that are IPv6 only and use CG-NAT.

If you think you don't 'need' IPv6: the Internet is currently in transition from v4 to v6 (for a couple of years) and every service should be "IPv6-ready" or rather IPv6-compatible. Disabling it on the server side is not a good plan for the future.

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There are no real disadvantages when disabling IPv6. As the Internet will have a need for both IPv4 and IPv6 for a very long time and all routers must / will support both.

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