When you are reading a
fully qualified domain name
(FQDN), which is a name that
includes the domain name (e.g.,
www.oreilly.com
, which includes the hostname
www
as well as the domain name
oreilly.com
), you start from the tail end. The rightmost
part of an FQDN is the top-level domain (TLD). The information related to the TLDs
is stored in root servers. If our DNS server wanted to look up
www.oreilly.com
, it
would start with the root server for the
.com
TLD. What it needs to do is to get the
server for
oreilly.com
. This process of iterative queries is called a
recursive querying
.
FQDNs can be hard to understand because the concept of a
domain name is sometimes difficult for people to grasp. A domain
name is sometimes used as a hostname itself, meaning it maps to
an IP address. Sometimes a name like
oreilly.com
may map to the
same IP address as the web server (e.g.,
www.oreilly.com
) but that
doesn’t mean they are always the same.
oreilly.com
is the domain
name. It can sometimes carry an IP address. A name like
www
or
mail
is a hostname and can be used all by itself with the right con‐
figuration. To be specific about which domain we are referring to
the hostname in, we use the FQDN including both the name of the
individual system (or IP address) as well as the domain that host
belongs to.
Once the DNS server has the root server for
.com
, it asks that server for information
related to
oreilly.com
. Once it has that name server, it issues another query to the
name server asking for information about
www.oreilly.com
. The server it is asking for
this information is the authoritative name server for the domain we are looking for.
When you ask for information from your server, what you will get back is a non-
authoritative answer. Although it originally came from an authoritative server, by the
time it gets to you, it’s passed through your local server so it is no longer considered
authoritative.
