What happens when a browser looks up a website

When you type a website name, the browser has to work out where to send the request. The Domain Name System, or DNS, helps translate a name into information the device can use to connect. The lookup often happens so quickly that you see only the page that follows.

Find the address behind the name

ICANN describes DNS as a way for people to use domain names instead of memorising numerical internet addresses. A domain name can have one or more address records. Your device or browser typically asks a DNS resolver to find the requested information.

A resolver can reuse an answer it already has. When it needs to follow the chain of authority, it can start with the root of DNS, find which servers handle the top-level ending such as .org, and follow the referrals to the authoritative servers for the name. Those are the servers responsible for answering for the relevant part of the naming system. A resolver can keep referrals as well as final answers, so a request often skips some of those steps.

Why one person can see a change before another

Imagine a publisher moves its website to a new server and updates its address record. Your resolver may already have the old address in its cache. Another visitor’s resolver might ask for a fresh copy and learn the new address sooner. Both visitors type the same name, but they may temporarily connect to different servers.

DNS records have a time to live, or TTL, which specifies in seconds how long the record can be cached under normal rules. If a resolver receives a record with a TTL of 600 seconds and the address changes a minute later, it may still have up to nine minutes of the original cache interval remaining. The clock follows when that resolver received its copy. It is possible for different caches to have different times remaining.

Separate the lookup from loading the page

Once the browser has usable connection information, it can contact the web server and request the page. A successful DNS answer leaves that next job to the connection and server. If the server returns an error, the lookup may already have done its job.

When reading about a site move or outage, look for which stage is affected. A changed address record, a failed connection and an error returned by a web server describe different events. Distinguishing them helps explain why the experience can vary from one visitor to the next.