In the referenced message, Eric A. Hall said:
Also, they don't have any special-case handling that I am aware of. I
tried to get a private /24 to use for the topology examples in my books
and couldn't get one. ARIN outright refused the request even though I
could prove the need for it, and even though I didn't care about global
routing or reachability.
Internet Assigned Numbers Authority (NET-TEST) IANA 192.0.2.0 - 192.0.2.255
The above, iirc, is the appropriate netblock to use in documentation
examples. So, they have already assigned you and every other author in the
world a /24 to use.