[billyhoffman]

Yep. RFC2606 It is what they should use. And if you need to specify 2 hosts, you can use example.net and .org as well.

Unfortunately, the example domains don't convey context very well, so we see things like target.com, victim.com, etc

[varikin]

This can be corrected by target.example.com and victim.example.com. Conveys the context while remaining safe as an example.

[pimlottc]

That generally works, although in some cases it makes a difference whether two hosts are on the same tld; at the very least, it implies a connection between the two that may not always make sense (why is aggressor.example.com attacking victim.example.com?).