Proper DNS for sub domains

Question

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?

Answer 1

Intermediate entries aren't necessary; if there are skipped levels then the DNS server before the skipped ones must know how to get to the DNS server after the skipped ones.

For instance, say you have d.c.b.a.com; The root servers know how to get to a.com; a.com could know how to get to c.b.a.com, and recursion would skip b.a.com altogether. It would then ask c.b.a.com's NS for the d record.