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At my job, IP prefixes with prefix length 31 (i.e. they only fit 2 IP addresses) are commonly described at "linknet addresses". Lately I have noticed that this term doesn't appear to exist outside of my company. If I do a Google search for "linknet address" or "link net address", I only find references to an ISP called "Linknet".

Does a term exist to describe this kind of IP prefix?

I'm asking because I'm developing a system where such IP prefixes are treated specially, so I need a term I can use to refer to them. I would prefer if this term wasn't one that was made up internally at my company.

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  • I had some luck doing searches for (in quotes) "a linknet" and "31 linknet". This reddit post has a few posters using "linknet" to refer to a /29, so it seems "linknet" isn't specific to /31s. Here's one that mentions a /27 linknet. (There are a few hits searching serverfault for "linknet", too.) – TylerW May 4 at 14:46
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A /31 is most often used for creating a point to point link between two networks, since it can do so by only using two addresses. Hence the name, 'linknet'

You can read this RFC for more information.

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    But curiously neither "linknet" nor "link net" appears in the RFS 🤷 So where does the term "linknet" come from? Is it defined anywhere? Is it simply a colloquial synonym for "point-to-point network address"? – Hubro May 3 at 15:25
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    It's commonly heard in network circles. See the first comment in this link as an example. – Bert May 3 at 16:14
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At former employer we'd have called this a "slash thirty-one" which saved two IPs over having to use a /30.

Nominally a /31 should not work, because the network and broadcast address take one IP each, leaving none for useful addressing. There were some items of customer equipment that couldn't handle it, so we had to use (or waste) a second /31 to make a /30.

The limitations on IPv4 availability make it practical to limit address wastage even a decade ago.

"linknet" would not have been used as a name, though if someone technical said "linknet" it would be obvious in context. A "point to point" link or network could have been said or used in documentation.

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    "There were some items of customer equipment that couldn't handle it" – Most notably, Windows, at least as recent as Windows 10 in 2019, does not implement RFC 3021 (which is from December 2000, i.e. before Microsoft reworked the network stack in Longhorn/Vista). – Jörg W Mittag May 5 at 6:06
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If it's specifically a /31 as opposed to a /30 (i.e. with only 2 addresses instead of the 4 normally required), then the usual term is a /31 (slash thirty one).

If it's just a subnet used for a point-to-point link, which happens to be a /31 because your equipment supports it, but could be a /30 in other cases, then "point-to-point network", "point-to-point net" is probably the generic term for that, though back in the days where you needed to use /30s and /31s where not an option, I believe we called them /30s (slash thirties).

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  • We sometimes call them "slash 31" and "get it together Microsoft" :-D – Jörg W Mittag May 5 at 6:08

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