4 part domain names

[Peter Risdon]

Jonathon McKitrick wrote:
> : Every unique combination of subdomain.domain.tld could point to an
> : arbitray other URL or IP.
> : For example
> : us.510.mail.example.com = example.com
> : de.510.mail.example.com = europe.mail.example.com
> 
> I guess my question is this...
> 
> if 'us' is the name of the node (machine) and 'example.com' is the
> registered domain name, what do the '510' and 'mail' parts uniquely
> identify?  Why not just 'us.example.com'?


I'm not an expert (IANAE should perhaps be a new acronym...), but here's 
how I understand it. The domain name system is completely open-ended and 
hierarchical. I tend to use djbdns for DNS servers, but found the bind 
concept of zones (and therefore zone files) very helpful.

The top level is . and this is managed by the top level name servers, 
which have to be hard-wired into any name server. These name servers 
delegate responsibility for the next level down to other name servers, 
so .com. (the trailing dot is left out for most purposes, but not all - 
see your hosts file as generated by sysinstall) is managed by a number 
of servers, .uk. by others, and so on. These name servers delegate 
authority for zones within their zones to yet more name servers. This is 
where we come in with .com. domains, but not .uk. where there is another 
layer of delegation before we ordinary mortals start managing delegated 
zones.

If you have registered example.com then authority for the whole zone of 
the internet, or the domain name system, below example.com. is delegated 
to you. You normally run name servers which publish information about 
hosts within this zone, but this is not the only thing you can do. You 
can also delegate authority for zones within this zone. Thus, there is a 
private company in the UK that delegates authority for zones below 
.uk.com. and thereby operates as a sort of private domain name registry.

So you could delegate authority to stated nameservers for a zone such as 
  mailservers.example.com and host information (A records) could be 
published by them for hosts such as smtp.mailservers.example.com. Of 
course, some of these could be aliases, and point to other hosts, but 
they don't have to be.

And so it goes on; there's no limit to the possible delegation of zones 
beyond common sense and convenience. The fact that we normally manage 
second level domains is a function of normal practice, it isn't 
intrinsic to the system and it isn't the case in the UK where we 
normally manage third level domains.

A DNS lookup for smtp.mailservers.example.com. would run as follows:

1. Look up in the static table of root name servers at least one value 
for a namserver that is authoritiative for .
2. Ask the . nameserver who is authoritative for .com.
3. Ask the .com. nameserver who is authoritative for example.com.
4. Ask the example.com. nameserver who is authoritative for 
mailservers.example.com.
5. Ask the mailservers.example.com. nameserver for the ip address of the 
host smtp.mailservers.example.com.

The following article explains how to delegate sub domains to name 
servers using bind. I can't find an equivalent for djbdns and suspect 
there might be a limitation in that software:

http://www.zytrax.com/books/dns/ch9/delegate.html



Corrections welcome...

Peter.

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