[zaarn]

These companies are the scum of the scum, tbh, I recall I once got a letter claiming I must pay about 6000€ for illegally downloading "Debian 5 Linux Netboot ISO" and "Ubuntu 12.04 x86 Full ISO" or something along those lines.

They sent some awfully scary letters for what amounts to legally obtaining an ISO file.

[notzorbo3]

I used to run an abandoned warez site when I was young. I received a lot of cease and desist letters from "lawyers". They usually failed to identify the infringing material, failed to show they had the right to act on the copywriters behalf and a staggering amount of them confused trademark infringement with copyright infringement. Also, every last one I received via email. Yeah, right, like that's going to hold up. I ignored all of them and never got even so much as a follow up.

In other words, such things are considered low-hanging fruit by these companies. Just throw it out there and see what sticks.

[zaarn]

Luckily the german system is less strict than the DMCA, you can fact-check any letters you get, you only need to act if you know (for certain) it's illegal

[floatboth]

How do these companies know who to contact for a given IP address? Do ISPs just cooperate with them?

[zaarn]

WHOIS records.

Though that only tells you the RIPE member for it.

They might have still leased the IP elsewhere (which is totally informal, if you operate a IP range all you need is a letter saying you can do it with the owner's signature)