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Site-Local/Unique-Local Addressing (IPv6)

On Mon, Jan 08, 2018 at 05:03:14PM +0000, Nicholas Warren wrote:
> Layman here, I was reviewing RFCs for a local address for IPv6. I came across two RFCs that seem interesting.
> 3879 Which deprecates Site Local Addresses.
> 4193 Which seems to add Unique Local Addresses.
> What is the main difference here? Why was this standard removed then added back?

Site Local Addresses are/were Site Scope, similar to how Link-Local are Link-Local Scope and others are Global Scope.

ULA are Global Scope--but that doesn't mean they are globally routable.

The problem with Scopes being built-in to the addressing model is that software has to be coded to treat different scopes differently.  It is hard enough to deal with Link-Local scope, and it was deemed too hard to deal with yet another scope--Site Local.  For an example of the pain, try using Link-Local addresses in a web browser, or even with "ping" on the command line.