A Domain Name System (DNS) translates a domain name such as [www.example.com](http://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) ](https://en.wikipedia.org/wiki/Time_to_live) .
- 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](http://www.example.com/)) or to an `A`record.
Services such as [CloudFlare](https://www.cloudflare.com/dns/) and [Route 53](https://aws.amazon.com/route53/) provide managed DNS services. Some DNS services can route traffic through various methods:
- Accessing a DNS server introduces a slight delay, although mitigated by caching described above.
- DNS server management could be complex, although they are generally managed by [governments, ISPs, and large companies](http://superuser.com/questions/472695/who-controls-the-dns-servers/472729) .
- DNS services have recently come under DDoS attack, preventing users from accessing websites such as Twitter without knowing Twitter's IP address(es) .
