Prefetching, also known as DNS prefetching, is a web performance optimization technique that involves resolving a website’s domain name to its corresponding IP address before a user actually clicks on a link or requests the site. This proactive resolution is done in anticipation of the user’s actions, with the goal of reducing the latency associated with DNS (Domain Name System) resolution.
DNS resolution is the process of translating a human-readable domain name, such as “example.com,” into a machine-readable IP address, like “192.0.2.1.” This translation is necessary for the browser to connect to the server hosting the website. However, DNS resolution can introduce a delay, as the browser must query DNS servers to obtain the IP address before it can initiate a connection to the website. This delay can add several seconds to a site’s page load time, especially if the DNS server is slow to respond or if multiple DNS lookups are required.
By prefetching DNS information, a browser can perform the DNS resolution in advance, while the user is viewing the current page. This means that when the user clicks on a link or initiates a new request, the browser already has the IP address available, eliminating the need for a real-time DNS lookup and reducing the overall time it takes to load the new page.
Prefetching can be implemented by website developers using HTML tags or HTTP headers that instruct the browser to resolve specific domain names ahead of time. Browsers may also automatically prefetch DNS information for links that are visible in the viewport, based on heuristics that predict which links the user is likely to click on.
While prefetching can improve performance by reducing latency, it also has potential downsides, such as consuming additional network resources and potentially impacting privacy by resolving domains that the user may not ultimately visit. Therefore, it should be used judiciously, with consideration for the specific needs and context of the website and its users.