Actually, neither this one nor cidr.xyz is fully compliant with the RFCs. Although I don't recommend it for pretty obvious reasons (and most routers probably won't handle it correctly, either), at least in the pre-CIDR RFCs, there is no requirement that subnet bits be contiguous. (I'm actually not sure if the CIDR RFC's address this - I don't keep up with this stuff anymore, and more recent RFC's my finally have discouraged these literally mindfcking netmasks...)
As for IPv6, yuck - it's the most brain-dead nomenclature ever written - if we'd tried* to make IP addresses as impossible to use and communicate as possible, this is wheat we'd do.
Somewhat related but I feel really sad that IPv6 is so slow to be adopted.
IPv4 scarcity is going to increase centralization as it’s going to cost more than ever to even operate an AS.
It’s pretty much going to be impossible to run an AS as an individual too for the same reason as a /24, the smallest block that you can pretty much announce on the Internet already sells for 5k+.
"I feel really sad that IPv6 is so slow to be adopted."
IPv4 fixed addresses are a service, and their scarcity made them even more costly, so I don't see how IPv6 is becoming widely adopted anytime soon: it's conflict of interests plain and simple.
IPv6 is already over 1/3 of traffic in the US, due to cross 50% this year. There are holdouts like GCP where you have to pay for more v4 usage but v6 isn't available but really deployment is doing quite well compared to 5 years ago. Also do not expect Asia to try to hold onto v4. I.e. it's slow but it's not being sniffled for the vast majority of users.
At this point the main thing holding back IPv6 adoption is enterprises that don't want to deal with it and home users with routers that don't have v6 function by default.
v6 is a perfectly fine protocol. It just lacked hardware and the pressure of actually being scarce on resources. I don't think anyone thought it would take 25 years to hit 50% adoption but now that we are getting close to that point it doesn't really make sense to swap again for something that's not supposed to solve any more problems.
It's a perfectly fine protocol for 25 years ago; juust before mobile devices with cellular data connections, on-the-fly connection switching, IoT, wifi, roaming, extenders, mesh networking. https://news.ycombinator.com/item?id=20167686
Some years ago I used this exact website to calculate non-overlap subnets when at work where I manage AWS VPCs for multiple projects. It's manual task when bootstraping a project, but it get done fast.
https://cidr.xyz/