#IPv6 #ULA is Broken (in Dual-stack Networks)
https://blogs.infoblox.com/ipv6-coe/ula-is-broken-in-dual-stack-networks/
@ChrisUplus Ever since I found that it's possible to just buy a GUA subnet, I cannot see what the point of ULAs is for. For internal use, it's not important that the purchased GUA subnet could be rescinded, but rather than the purchased subnet is: 1) a GUA, thus scoring higher on the precedence table, and 2) the issuer won't ever issue that subnet to someone else that might interconnect to your internal network. This beats praying for the avoidance of ULA collisions.
Yes, "routed GUA > NPTv6+GUA > NPTv6+ULA"
Buying a GUA to reserve the address space, but not actually routing it (i.e. using NPT from your ISP) -- instead of a ULA -- is a nice trick (and fixes precedence issues).
However if you accept this, then there is a case for ULA when buying a GUA is not feasible.
e.g. Container (or virtual machine) network that is created on the fly.
No real chance to buy a GUA.
Working in software dev, I may spin up a small local virtual network with a few endpoints.
For this scenario you may use NAT66 if the devices need to call out (or NAT64 as relevant).
More likely access may be through a reverse proxy, e.g. one of the containers is one the internal network + port mapped.
I guess I could buy a GUA and reuse it; but what about if I want to put up an example for others to download and run -- can't really have step 1 "buy a GUA".
Same for a default internal network for something like VMWare.
Your external network could be bridged (so use the IPv6 range from your main network), but what about a private network?
(Again, you could buy a GUA and use that, but that isn't a useful default)
The cases for ULA are small, but do exist.
