!!! Overview
[{$pagename}] is defined in [OAuth 2.0 for Native Apps] ([RFC 8252]) section 7.3

Native apps that are able to open a port on the [loopback] network interface without needing special permissions (typically, those on desktop [Operating Systems]) can use the loopback interface to receive the [OAuth] redirect.

[Loopback] redirect [URIs] use the "[http|HTTP]" scheme and are constructed with the [loopback] [IP] literal and whatever port the client is listening on.

That is, 
%%prettify 
{{{
http://127.0.0.1:{port}/{path}
}}} 
/%
for [IPv4], and
%%prettify 
{{{
http://[::1]:{port}/{path}
}}} 
/%
for [IPv6].  An [example] redirect using the [IPv4] [loopback] interface with a randomly assigned port:
%%prettify 
{{{
http://127.0.0.1:51004/oauth2redirect/example-provider
}}} 
/%

An [example] redirect using the [IPv6] [loopback] interface with a randomly assigned port:
%%prettify 
{{{
http://[::1]:61023/oauth2redirect/example-provider
}}} 
/%
The [Authorization Server] [MUST] allow any port to be specified at the time of the request for [loopback] [IP] redirect [URIs], to accommodate clients that obtain an available [ephemeral] [port] from the [Operating System] at the time of the request.

Clients [SHOULD NOT] assume that the device supports a particular version of the [Internet Protocol].  It is [RECOMMENDED] that clients attempt to bind to the [loopback] interface using both [IPv4] and [IPv6]  and use whichever is available.

!! More Information
There might be more information for this subject on one of the following:
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