[urineaut]

That's not a language-specific thing, but is actually part of the IPv6 RFCs as IPv4-mapped IPv6 addresses: [1], [2]

This is super useful because (at least on Linux) IPv6 sockets per default are dual-stack and bind to both IPv6 and IPv6 (except if you are using the IPV6_V6ONLY sockopt or a sysctl), so you don't need to open and handle IPv4 and IPv6 sockets separately (well, maybe some extra code for logging/checking properly with the actual IPv4 address).

That is also documented in ipv6(7):

  IPv4 connections can be handled with the v6 API by using 
  v4-mapped-on-v6 address type; thus a program needs to support only
  this API type to support both protocols.  This is handled
  transparently by the address handling functions in the C library.
  
  IPv4 and IPv6 share the local port space.  When you get an IPv4
  connection or packet to an IPv6 socket, its source address will be
  mapped to v6.

[0]: https://datatracker.ietf.org/doc/html/rfc5156#section-2.2 [1]: https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5....