When you enter a website’s address into a browser, the browser looks for the IP address of the website. The first place the browser checks is from your Internet Server Providers (ISP) records. If the site is not listed in the records it queries registrars to find out who the DNS start of authority (SOA) is for the website. If the domain is using it’s registrar’s name server as it’s SOA, the browser looks up the “A” record for your domain and returns the IP address of the server listed. If the domain is using a hosting service’s name servers, the registrar points the browser to the host’s DNS servers to determine the IP Address for the domain name. From there the request is sent to the server the domain is hosted on which then provides the browser with the website.
To speed the loading of websites, each ISP caches a copy of DNS records for a period of time, sometimes up to 48 hours. This means that they make their own copy of the registrars’ master DNS records, and reads from them locally instead of making a direct request to the domain registrar every time a request is made. This speeds up web surfing quite a bit by:
- decreasing the return time it takes for a web browser to request a domain lookup and get an answer and
- reducing the amount of traffic on the web.
The downside to caching the master DNS records is because each company or ISP only updates their records every few days, any changes you make to your DNS records are not reflected between those updates. Although some DNS servers update every few minutes, the time between updates system wide is not standardized so the delay can range from a few hours to several days. This slow updating of the cached records is called propagation delay because a website’s DNS information is being propagated across all DNS servers on the web.