What Happens When You Open a Website?
A tour through DNS, connections, encryption, HTTP and the browser's rendering process.
From a name to a destination
When a web address is entered, the browser first identifies the host name. It needs an IP address to contact, so it consults DNS. Answers may come from the browser cache, the operating system, a local network resolver or a recursive DNS service.
DNS is a distributed naming system. It allows people to use memorable names while networks use numerical addresses to deliver packets.
Establishing a connection
The browser then establishes a connection to the server. Depending on the protocol and network, this may use TCP or QUIC. For a secure HTTPS session, TLS provides encryption and authentication.
During TLS setup, the browser and server agree on cryptographic parameters and establish shared session keys. The browser also checks the server certificate against the requested host name and its trusted certificate authorities.
HTTP carries the request
HTTP defines how clients and servers exchange requests and responses. A request includes a method, a target resource and headers. The response includes a status code, headers and usually a body.
A successful response does not guarantee that every part of a page will load. The HTML may refer to a stylesheet or image that is missing, blocked or slow. Browsers handle these resources separately.
Rendering the page
The browser parses HTML into a document structure and CSS into style rules. It calculates layout, paints pixels and runs scripts when required. These processes can overlap, so visitors may see the page before every resource has finished loading.
The browser also applies security rules, manages storage and may reuse cached content. These details help make repeat visits faster and protect users from some classes of unsafe behaviour.
Diagnosing a failure
The sequence provides a practical troubleshooting guide. If a domain cannot be resolved, inspect DNS. If a connection cannot be established, inspect routing, firewall rules and server availability. If HTTPS reports a certificate error, check the certificate chain, host name and system clock. If only one part of a page is missing, inspect that resource's request and response.