4 part domain names

[Hexren]

JM> : Every unique combination of subdomain.domain.tld could point to an
JM> : arbitray other URL or IP.
JM> : For example
JM> : us.510.mail.example.com = example.com
JM> : de.510.mail.example.com = europe.mail.example.com

JM> I guess my question is this...

JM> if 'us' is the name of the node (machine) and 'example.com' is the
JM> registered domain name, what do the '510' and 'mail' parts uniquely
JM> identify?  Why not just 'us.example.com'?


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Imagine not beeing a person with very simple needs like me or you but
a big corporation. The corp where I work for example has adresses like
uws015.ham.example.com and uws015.fra.example.com uws015 stands for
"unix workstation number 15" ham stands for
Hamburg and fra for Frankfurt (actually these are 3 letter airport codes as
I'm employed by an airline) that makes sense as you now can delegate
the numbering of workstations to the local sysadmins in Hamburg and
Frankfurt. Whichout them having to worry about taking a number that is
already used.

For your question it is very likely that example.com is a big webmail
provider it would therefore make sense for them to have many different
login servers all over the world. I could guess that mail identifies a
mail server (big surprise there) and us.mail.example.com would
therefore be a mail server in the United States whereas
de.mail.example.com would be a mail server in Germany. Why they use
mail.example.com, because they maybe also have us.news.example.com used. So it
makes sense to group their many servers by function first and then by
location. 510 could identify a rack or a datacenter so that
us.510.mail.example.com means "a mail server in the datecenter with
the id 510 which serves the United States".
But be aware that that extra meaning is arbitrary choosen by the Sysadmins at example.com
they could aswell setup something like only.dump.persons.login.here.example.com and
point their accountants to this server for login purposes. So if you
are interested into why some specific specific subdomain names where
choosen it would be wissest to ask the Sysadmins wo administer DNS for
that domain.

Hexren