[usr1106]

Well that's true. Probably there are users that use it in violation of the spec, relying on that it would not harm.

Was it 1.1.1.1 that had quite some problems in the beginning of their operation or some similar one? I vaguely remember reading a blog post at the time.

[zamadatix]

For 240.0.0.0/4 it's not as much existing users violating spec as existing in-spec software and hardware not allowing it. E.g. even if you patched your Linux box and DHCP server to support 224.0.0.0 your hardware router might not forward the packet between zones, your Windows clients might not accept the assignment. In the public case your ISP might not accept it in their router hardware or filters and even if they did it doesn't mean the other 100,000 entities on the internet you're trying to talk to/through do. The same is all true with 224.0.0.0/4 as well plus the fact there is existing in spec use for multicast.

1.1.1.1 was never reserved but it was unassigned until 2010. By that point it had been used improperly so much it received massive amounts of garbage data when advertised (and still does to this day). It's just that "massive" turns to "quite tiny" in context of a giant CDN like Cloudflare so they were able to salvage it.

[pests]

Yes it was 1.1.1.1. I remember the initial blog post. Before even turning DNS on they just monitored traffic patterns and types to make sure they could handle it.

[sgjohnson]

That’s actually exactly the reason why Cloudflare got it. They were the only ones at that point who could handle all the garbage that was sent to it, and willing to deal with it at their own expense.