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For more than four days, a server at the very core of the Internet’s domain name system was out of sync with its 12 root server peers due to an unexplained glitch that could have caused stability and security problems worldwide. This server, maintained by Internet carrier Cogent Communications, is one of the 13 root servers that provision the Internet’s root zone, which sits at the top of the hierarchical distributed database known as the domain name system, or DNS.

Here’s a simplified recap of the way the domain name system works and how root servers fit in:

When someone enters wikipedia.org in their browser, the servers handling the request first must translate the human-friendly domain name into an IP address. This is where the domain name system comes in. The first step in the DNS process is the browser queries the local stub resolver in the local operating system. The stub resolver forwards the query to a recursive resolver, which may be provided by the user’s ISP or a service such as 1.1.1.1 or 8.8.8.8 from Cloudflare and Google, respectively.