Mastodawn

Lukas Jul 18, 2025

Mildly cursed factoid about UNC paths:

- UNC Paths can contain IP addresses such as \\192.168.1.1\share
- IPv6 addresses are supported as well
- IPv6 addresses contain colons
- can't have colons in Windows paths since colons are reserved for drive letters

So Microsoft came up with the the ipv6-literal.net domain that's special-cased by Windows so you can to write IPv6 addresses in UNC paths as 2a0e-3c0--21.ipv6-literal.net without it hitting any resolvers.

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Lukas Jul 18, 2025

Turns out, it's worse than this: The ipv6-literal.net shenanigans aren't limited to UNC paths.

It works in all places:
> ping 2a0e-3c0--21.ipv6-literal.net

Pinging 2a0e:3c0::21 with 32 bytes of data:

Also works in browsers: http://2a0e-3c0--21.ipv6-literal.net/

Thanks to @casandro for asking.

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Axel 🚴😷🐧🐪⌨ | #WeAreNatenom

@karotte @casandro: The domain ipv6-literal.net doesn't look as if it belongs to Microsoft (as @GossiTheDog already mentioned in his posting). It is parked at GoDaddy.

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Christian Huitema Jul 18, 2025

@xtaran @karotte @casandro @GossiTheDog

That's an old one! We came up with it for IPv6 when I was working at Microsoft. I think it was introduced in Windows XP SP1. We were worried that typing addresses like 2001:2:3::abcd was not going to work well in a variety of user interfaces, hence the "2001-2-3--abcd.ipv6-literal.net" hack. One of the team members (maybe me, not sure now) registered it on a personal account, and then transferred ownership to Microsoft before shipping.