A part of the zone file is set up by the registrar, that is, the DNS servers for a domain. However, other parts of the zone file are configured by a DNS server. These parts include the “A” records, “MX” records and “CNAME” records.
Via the use of “A” records, one can define subdomains.
Note that you can choose whether to implement your own domain server. If you chooose not to implement your own DNS server, you can use a DNS hosting service. There are many free DNS hosting services. Many DNS hosting services have multiple servers to ensure availability. Because DNS serving is the first link of most network based services, an organization cannot afford to lose DNS service.