A top-level domain is one that has exactly one component, e.g., "com",
"org", "uk", or "tv". We also talk about suffixes, which would probably
include "co.uk", "k12.ca.us", and "edu.au". We mandate not querying the
top-level domains, since they can be algorithmically determined and we
really don't want to unnecessarily load the TLD servers. Not querying
suffixes is optional, as the definition for what a "suffix" is, because
there is no formal definition and this is really an optimization.
The abuse.net database lookup does an internal walk up the tree
looking for an entry (in its own database, not the DNS), and I can
report from experience that trying to track delegation point suffixes
is an extremely hard problem. Things that look parallel often aren't,
e.g., in the various state k12.xx.us domains, in some states there is
one agency that serves all the schools so it's a real domain that
should have a record, while in others it's just a label. And the list
is highly dynamic, e.g., sakhalin.ru used to be a geographic label,
similar to a lot of other <place>.ru domains, but now they have a
municipal ISP and it has an MX record.
R's,
John
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