It can be anything! An apex, subdomain, or even a URL.

Results forns.tk

Resolver

Location

NS

Name	TTL	Value
ns.tk	21600	a.ns.tk joost\.zuurbier.dot.tk 2017063100 14400 3600 604800 86400

Name	TTL	Value
ns.tk	21600	v=spf1 -all