Domain Name Service (DNS)
Domain Name Service is a distributed directory of the Internet that
comprises records which define how domain names are directed to
services. Examples include A records that point to numeric IPv4
addresses, AAAA records (IPv6 addresses), MX records (mail servers)
and CNAME records (aliases).
Servers that respond to DNS requests may be recursive resolvers, root name servers or authorized name servers.
The records stored in root name servers (such as a.root-servers.net)
define the top level domain name servers, such as a.fi, which is one
of many name servers for the fi top level domain. The records in
these servers define all authorized name servers underneath, such as
ns1.208.fi and ns2.208.fi, which are the authorized name servers
for some fi domain names, such as 208.fi and
projectorganisationhelsinki.fi.
Recursive resolvers form a kind of distributed cache. For example, the name server in a home router will respond to requests from the devices in the local area network. It may further request records from a name server that is operated by the Internet Service Provider (ISP). Ultimately, if needed, the records will be requested from authorized name servers, which will be determined by following the chain starting from one of the root name servers.
Private DNS
Virtual private networks may comprise name servers that are distinct from the Internet, responding in a different way to some requests. For example, there is no domain name
secret.208.fiin the public Internet, but there might be one in the virtual private network of our company. Such name servers could fetch external names as is from the public DNS, optionally filtered. For example, the namecompetitor.example.comcould be redirected to a different address.