I'm surprised how few people are talking about ULAs. For any home network where you don't have a reserved global address space from your ISP, it makes sense to configure a ULA on your router and use it for all internal hosts, and the ISP assigned address is only used for Internet access.
This does not require NAT/Npt and you have the best of both worlds.
Unmentioned in the Wikipedia article is RFC 7368 IPv6 Home Networking Architecture Principles[1] that discusses them as well.
> A home network running IPv6 should deploy ULAs alongside its globally unique prefix(es) to allow stable communication between devices (on different subnets) within the homenet
I use ULAs since my ISP changed my ipv6 prefix twice a month. What I learned is that ipv6 configuration usually requires a more 'comerical grade' router. In the end it works well for me but getting it setup initially took a lot of effort. Easy to do the second time though.
I later moved and my current ISP does not have ipv6 support but my ULA setup kept working fine with some minor tweaks.
RFC 7368 for home networks recommends the use of ULA locally.
> A home network running IPv6 should deploy ULAs alongside its globally unique prefix(es) to allow stable communication between devices (on different subnets) within the homenet
> When an IPv6 node in a homenet has both a ULA and a globally unique IPv6 address, it should only use its ULA address internally and use its additional globally unique IPv6 address as a source address for external communications.
RFC 7368 is a 2014 "informational" (no ietf standing) doc so it's not a source for current IETF advice. Also it was part of the since closed "homenet" working group initiative trying to define some new stuff that did not get vendor uptake.
But in substance, if you have several subnets, then using ULA may make sense depending on what you're trying to do. However most home networks don't subnet.
It’s pretty sweet. By using ULA addresses for everything, all internal networking keeps working as-is if my ISP allocation changes. Every host can talk to its neighbors using internal addresses, and still connect to remote hosts without NAT breakage.
mDNS is orthogonal to ULA. mDNS is for discovery and name resolution, whereas ULA is for IP connectivity. And mDNS operates at the link-local scope (link-local addresses), whereas ULA is scoped for the entire home network.
When a Multicast DNS responder sends a Multicast DNS response message
containing its own address records, it MUST include all addresses
that are valid on the interface on which it is sending the message,
and MUST NOT include addresses that are not valid on that interface
(such as addresses that may be configured on the host's other
interfaces). For example, if an interface has both an IPv6 link-
local and an IPv6 routable address, both should be included in the
response message so that queriers receive both and can make their own
choice about which to use. This allows a querier that only has an
IPv6 link-local address to connect to the link-local address, and a
different querier that has an IPv6 routable address to connect to the
IPv6 routable address instead.
So instead of using static ULA addresses, you can use the the routable address returned by mDNS. It can often replace the ULA address use case.