DNS Zones | Networking Services | Networking Fundamentals | CS School


The DNS resolves or translates the name into IP addresses anytime we want to access resources over a network. Because we use IP network, therefore name actually doesn’t work over the network. We already have covered Name Resolution Steps as well as DNS Resource Record Type. Here we will discuss about the concept of DNS Zones, and how this allow easier control over multiple levels of a domain.

We know that Authoritative_Name_Servers are responsible for responding to name resolution requests for specific domains. But an Authoritative_Name_Server is also responsible for a specific DNS Zone.

The DNS Zones

DNS Zones | Networking Services | Networking Fundamentals | CS School

DNS Zone is a hierarchical concept. The root name servers are responsible root zones, each TLD name server is responsible for Specific TLD zone. Hence the authoritative name servers are responsible for specific DNS Zone that returns the final IP address of the asked domain. Although the Root and TLD name servers are also like authoritative name server, but it is just that the zones that they are authoritative for are special cases.

It is the fact that the zones don’t overlap. For example the administrative authority of the TLD name server for the .com TLD doesn’t encompasses microsoft.com. Instead it responds with the authoritative name server responsible for microsoft.com. The reason of configuring DNS zones is that it allow easier control over multiple levels of a domain. As the number of resource record increases for a single domain, it becomes difficult to handle all of them. Therefore creating multiple zones splitting up the configuration for each resource record network engineers has eased this pain.

How DNS Zone made easier control over multiple levels of Domain

For example, a company’s domain is somecompany.com. This company has offices in different region in the world like india, china, and Bangladesh. Also consider that each regional office of the company has 200 uniquely named desktop computers. So it needs 600 A records in a single DNS zone to keep track of all of the computers. Instead of this the company could split up their office into their own specific zone. So the setup will be inn.somecompany.com, cnn.somecompany.com, and bnn.somecompany.com all subdomains under somecompany.com. Here each subdomain will be with their own specific DNS zones. Now for this setup a total of four authoritative name servers will be necessary, one for somecompany.com and one for each of the subdomains.

Zone Files

A zone file is a simple configuration file that declares all resource records for a particular zone. A zone file must contains an SOA record. Here SOA stands for Start of Authority. So a zone file has to contain Start of Authority resource record declaration. The SOA declares the zone and the name of the name server which is authoritative for it. Along with SOA record a zone file also contains NS records. It indicates other name servers that might also be responsible for this zone.

Beside SOA and NS records the zone files may also contain other resource record such as A Record, Quad A Record, CNAME Record etc. In the zone file there is also other configuration such as default TTL values for the records served by the zone.

Reverse Lookup Zone files

DNS zone can also contain reverse lookup zone files. These files let DNS resolvers ask for an IP and get the associated FQDN in return. The reverse lookup zone files are as same as zone files except, it resolves IP to a name similar to PTR or pointer resource record that resolves an IP to a name.

For simplicity’s sake here we have considered a single server what is responsible for its zone, whether at the root, TLD or domain level. But there is often multiple physical server with their own FQDNs and IP addresses involved. For something as important as DNS having multiple servers is pretty common. Because if one server suffers a hardware failure, there is always another server to handle the DNS traffic.

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