I saw a subdomain of a subdomain: meta.unix.stackexchange.com
That is normal, a fully qualified domain name (FQDN) can have a large number of elements. The DNS protocol and most (probably all) DNS server implementations allow you to delegate subdomains to arbitrary depth. I think until you hit the 255-character hard limit on the length of a FQDN. each element must be no more than 63 characters long.
For example DNS would let some eccentric admin create
And still have "A" records at intermediate levels.
How is it possible?
The way I used to create subdomains when I ran the top level of a corporate DNS service was to delegate subdomains, often to other servers but sometimes to the same nameserver but keeping the subdomain data separated into distinct zonefiles. This was using BIND.
I would never have dreamed of using a dot in a node name.
The following syntax will result in fewer problems with many applications
that use domain names (e.g., mail, TELNET).
<domain> ::= <subdomain> | " "
<subdomain> ::= <label> | <subdomain> "." <label>
<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
<let-dig-hyp> ::= <let-dig> | "-"
<let-dig> ::= <letter> | <digit>
<letter> ::= any one of the 52 alphabetic characters A through Z
in upper case and a through z in lower case
<digit> ::= any one of the ten digits 0 through 9
Note that this does not provide for dots in labels. Software that supports this is probably effectively creating subdomains under the hood.
- DOMAIN NAME SPACE and RESOURCE RECORDS
3.1. Name space specifications and terminology
The domain name space is a tree structure. Each node and leaf on the
tree corresponds to a resource set (which may be empty). The domain
system makes no distinctions between the uses of the interior nodes and
leaves, and this memo uses the term "node" to refer to both.
Each node has a label, which is zero to 63 octets in length. Brother
nodes may not have the same label, although the same label can be used
for nodes which are not brothers. One label is reserved, and that is
the null (i.e., zero length) label used for the root.