[ih] The classful <net>.<host> IPv4 address format

[Craig Partridge]

IEN 21 (TCP v. 3) specifies a network.host.port format, which I suspect is
the origin of the net.host form.

https://www.rfc-editor.org/ien/ien21.pdf

Craig

On Sun, Sep 14, 2025 at 6:21 AM Michael Kjörling via Internet-history <
internet-history at elists.isoc.org> wrote:

> Every once in a while I see people being surprised that IPv4 addresses
> can be expressed in formats other than dotted-decimal-quad; more
> specifically, in classful-style <net>.<host> (where net and host can
> add up to less than four octets) or even single-large-integer format.
>
> I am _almost_ certain that I have seen an early IP or TCP RFC which
> actually describes this representation, but have been unable to locate
> that.
>
> Can anyone point me at an authoritative source where the
> dotted-not-quad textual IPv4 address representation format is defined
> or at least described? Thanks!
>
> --
> Michael Kjörling
> 🔗 https://michael.kjorling.se
>
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