# Domain Name System

A Domain Name System (DNS) translates a domain name such as www.example.com to an IP address.

DNS is hierarchical, with a few authoritative servers at the top level. Your router or ISP provides information about which DNS server(s) to contact when doing a lookup. Lower level DNS servers cache mappings, which could become stale due to DNS propagation delays. DNS results can also be cached by your browser or OS for a certain period of time, determined by the time to live (TTL).

- NS record (name server) - Specifies the DNS servers for your domain/subdomain.
- MX record (mail exchange) - Specifies the mail servers for accepting messages.
- A record (address) - Points a name to an IP address.
- CNAME (canonical) - Points a name to another name or CNAME (example.com to www.example.com) or to an A record.

Services such as [CloudFlare](https://www.cloudflare.com/dns/) and [Route53](https://aws.amazon.com/route53/) provide managed DNS services. Some DNS services can route traffic through various methods:

- [Weighted Round Robin](https://www.jscape.com/blog/load-balancing-algorithms)
  - Prevent traffic from going to servers under maintenance
  - Balance between varying cluster sizes
  - A/B testing
- [Latency Based](https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/routing-policy.html#routing-policy-latency)
- [Geolocation Based](https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/routing-policy.html#routing-policy-geo) 

To learn more, visit the following links:

- [Getting started with Domain Name System](https://github.com/donnemartin/system-design-primer#domain-name-system)
[What is DNS?](https://www.cloudflare.com/learning/dns/what-is-dns/)