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To: "cygwin@cygwin.com" <cygwin@cygwin.com>
Subject: RE: [EXTERNAL] Re: Please support download setup-x86_64.exe on
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Thread-Topic: [EXTERNAL] Re: Please support download setup-x86_64.exe on
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Date: Tue, 21 Nov 2023 18:11:42 +0000
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From: "Lavrentiev, Anton \(NIH/NLM/NCBI\) \[C\] via Cygwin" <cygwin@cygwin.com>
Reply-To: "Lavrentiev, Anton \(NIH/NLM/NCBI\) \[C\]" <lavr@ncbi.nlm.nih.gov>
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> ::ffff:0:0:0/96 == ::ffff:0:a.b.c.d

https://www.ibm.com/docs/en/zos/2.1.0?topic=addresses-ipv4-mapped-ipv6

Mapped IPv4 addresses have the ::ffff:a.b.c.d short form, without any intervening 0 word.  The CIDR form above just denotes that 96 bits
are the prefix (the "network" part) and, thus, the remaining 32 bits is the host part within the network.  IPv6 addresses are 128 bit long.

But the ::ffff:0:a.b.c.d notation is not a mapped IPv4 address, it's just some IPv6 address which is written with last
four octets in the IPv4 notation (any IPv6 address can be accepted this way), but when you convert it back from its binary
form, that won't re-emerge the same way because only IPv4 compatible (all 96 leading bits zero) or IPv4 mapped (80 zero bits
followed by 16 one bits) followed by 4 bytes of IPv4 address) can be conventionally written in the IPv4 form (depending
on the system settings).

E.g. 1.2.3.4 (input) -> (binary rep) ::ffff:1.2.3.4 -> (output) 1.2.3.4

but

::ffff:0:1.2.3.4 -> (binary rep, same) -> (output) ::ffff:0:102:304

HTH,

Anton Lavrentiev
Contractor NIH/NLM/NCBI


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