The Domain Name Service, or DNS, is one of the most important contributing factors to the widespread popularity and accessibility of the Internet. DNS is essentially a phone book for the internet. It hosts a list of easy-to-use names and labels that translate into the actual physical routing addresses for different spots on the internet. It translates what are commonly known as ‘domain names’ (www.example.com) into their IP address counterparts (192.0.2.19, for example) and vice versa. 

A domain name consists of two or more parts separated by a dot (www.example.com). The rightmost part (.com) is known as the top-level domain. Every section to the left of that is subdivision, or subdomain, of that. Theoretically, a domain can have 127 subdomains, but most domain registries have shorter limits.

 

The servers that are responsible with keeping up the various records of which IP addresses resolve to which domain names are known as nameservers. In the technical model of the DNS process, a request for the IP address of a given domain name, starts first with a request to the root nameservers, the addresses of which are maintained directly on the local system by its DNS client. The root nameserver refers the client to an authoritative server for the next level down. An authoritative server is one that is directly responsible for a set of domains. The process repeats for each level of the domain, until the full domain is found and the IP address for it is retrieved.

 

Each authoritative domain server has a set of zone files. A DNS zone file is a database in the DNS system that is used to store the exact mappings from one domain name to a given IP address or other resource. These mappings in the zone file are called DNS records. The most common type of record is known as an ‘A’ record, which contains the mapping of a domain name to an IP address. Other type of DNS records include ‘MX’ records, which map the mail exchange server for a given domain, or a ‘TXT’ record, which stores a text string, often for use with protocols such as Sender Policy Framework or DomainKeys.

 

An important thing to keep in mind when assigning or changing the DNS for a domain or IP Address is the time delay involved. Known as DNS propagation, it stems from the practice of caching DNS lookups on a local system, and keeping them for a set amount of time to try to reduce the load on the DNS servers. However, this practice creates a problem if DNS records are updated, as any systems or servers that have the old information cached will still be trying to access the old addresses and information instead of the updated records. The duration that a record will remain cached is known as its Time to Live (TTL) which typically lies between 3-24 hours for most records. Once this period has passed, all of the records will have been retrieved with the change, and everyone accessing a resource will be directed to the proper place.