The Domain Name System (DNS) is the internet's address book. It translates human-readable domain names like "example.com" into IP addresses like 93.184.216.34 that computers use to connect to each other. Without DNS, you'd have to memorize IP addresses for every website you visit.
How DNS works
When you type a domain name in your browser, your computer sends a DNS query to a resolver (usually your ISP's server or a public one like 8.8.8.8). The resolver checks its cache first. If it doesn't have the answer, it walks up the DNS hierarchy: root servers point to TLD servers, which point to the domain's authoritative nameservers, which finally return the IP address.
The whole process takes milliseconds. DNS records come in several types. A records map a domain to an IPv4 address. AAAA records handle IPv6. MX records point to mail servers. CNAME records create aliases. Each record has a TTL (time to live) that tells resolvers how long to cache the answer.
What happens to DNS when a domain expires
When a domain expires and gets deleted, its DNS records are wiped from the registry. Query that domain and you'll get an NXDOMAIN response, meaning "this domain doesn't exist." That's actually useful. CatchDoms uses mass DNS checks to scan millions of domains from Common Crawl data. If a domain returns NXDOMAIN, it's potentially deleted and available for registration. This is the first filter in the regfree pipeline that finds deleted domains across 49 country-code TLDs.