To begin, I am going to describe my organisation domain set up which may be common or uncommmon to everyone else, I am not sure since I am inexperienced -- but here goes:
We have the domain company.co.uk which is a registered domain i.e. you can do an nslookup on it. Incidentally, our organisation is physically separated at 2 different places hence our AD and other servers are actually set up under country1.company.co.uk and country2.company.co.uk like so
primary-dc.country1.company.co.uk secondary-dc.country1.company.co.uk some-service.country1.company.co.uk primary-dc.country2.company.co.uk secondary-dc.country2.company.co.uk some-service.country2.company.co.uk
the third party web host machine at the IP of company.co.uk's A record is the machine for the authorative DNS (and web server), has its usual A and NS records, and records for the services which can be accessed externally, for example mail and VPN.
QUESTION (finally): I realized that a single A Record for some-service.countryN.company.co.uk, defined at company.co.uk, seems to be sufficient for the internet to find its way to our LAN server. But, is it really sufficient?
As some-service is a 4th-level domain (or is it 5th?), it surprises me when it even resolves as there are no entries for the intermediate levels. Why is this so? Or do I need to also set the NS for some-service at least?