[dathinab]

I'm not completely certain, but my guess is a lot of software and midleware doesn't work well with non-continuous subnet masks.

It's one of this (many) thinks where in theory the RFC has intended support for this but in praxis it's close to unusable and better to block for most application use cases.

Like e.g. many valid email-addresses are in practice unusable and it's recommended to not support them because today they are basically only used to intentionally cause problems (I mean email addresses using quoting like `"a b"@example.com`, you always should support internationalized mail addresses, except maybe as mail provider where it depends on your target customers).