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This post asks two questions, a primary and related question.

I have been told, perhaps erroneously, that it is wisest to keep IPV6 turned on, especially on newer versions of Windows Server. To the best of my knowledge, nothing in our Intranet takes advantage of IPV6.

My question is, what is the downside to unchecking IPV6 on the adapter configuration screen?

My second question is rhetorical. If these servers all have static IP4 addresses, then I am assuming it could screw things up if the IPV6 configuration is set up as DHCP, or does the Windows network just ignore that?

This mismatch is what I discovered today, and I just re-configured the IPV6 information to have an IPV6 static address for the adapter with the same DNS and gateway entries (represented as IPV6 addresses) as used for IPV4.

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  • "I just re-configured the IPV6 information to have the same information as the IPV4 (with IPV6 address conversions, of course)." You need to explain that. IPv4 and IPv6 are completely separate protocols, and there is no direct conversion between them. Windows itself uses IPv6 internally. Disabling it on an adapter doesn't disable IPv6 in Windows. There is a way to do that, but Microsoft will not provide support if you completely disable IPv6 in Windows.
    – Ron Maupin
    Commented Jul 10, 2017 at 17:40
  • @RonMaupin I've re-worded the post. I took the static address for ipv4 1.2.3.4, converted it to IPV6, and used the converted value. I did the same for gateway and two DNS servers. Thanks for the answer; suggest you post your comment as an answer.
    – octopusgrabbus
    Commented Jul 10, 2017 at 18:01
  • You can't do that. IPv4 and IPv6 addressing are not compatible. You must use that actual IPv6 addressing, which has nothing to do with the IPv4 addressing.
    – Ron Maupin
    Commented Jul 10, 2017 at 18:03
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    @RonMaupin You mean you can't convert the IP4 addresses?
    – octopusgrabbus
    Commented Jul 10, 2017 at 18:04
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    Right. IPv4 addresses are 32-bit integers, but IPv6 addresses are 128-bit integers. IPv6 uses a link-local IPv6 address on every interface, and it can use multiple global (something you cannot make up) and/or ULA (has special rules for creating) addresses on the interfaces, too. None of this has anything to do with the IPv4 addresses.
    – Ron Maupin
    Commented Jul 10, 2017 at 18:06

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My question is, what is the downside to unchecking IPV6 on the adapter configuration screen?

If you don't use IPv6 in any way on this interface, then almost nothing happens when you disable it on the interface. You can check with Wireshark if there is any useful IPv6 traffic if you want to be 100% sure though. Don't be afraid of any control protocols, like ICMPv6, NDP, MDNS etc, only look for data traffic encapsulated (wich is most probably encapsulated in TCP or UDP).

My second question is rhetorical. If these servers all have static IP4 addresses, then I am assuming it could screw things up if the IPV6 configuration is set up as DHCP, or does the Windows network just ignore that?

IPv6 settings cannot affect IPv4 reachability in any way. Something else can change though when you assign routed IPv6 addresses to hosts: modern OSes prefer IPv6 over IPv4 when a target host is accessible by both IPv6 and IPv4. This can be influenced, but default behavior is to prefer IPv6. If your hosts register in DNS their IPv6 addresses, then most likely they all will use IPv6 for connectivity.

The last point is IPv6 has several methods of automatic assigning of addresses, and DHCPv6 is only one of them. If you just leave the interface settings in "Obtain an IPv6 address automatically" position, and you don't have IPv6-enabled router in your network, then the hosts will generate link-local addresses (something similar to APIPA in IPv4 world). These addresses can be used for communications even though nothing was configured.

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