What Is DNS and How Does It Work? A Practical Guide to the Domain Name System

Every time you type a domain name into a browser, a DNS lookup happens before the page can even start loading: the name has to be translated into an IP address. DNS (Domain Name System) performs that translation, and when it breaks, the site is unreachable no matter how healthy the web server is.

The path of a DNS query

• Browser and OS cache: if the answer was fetched recently, the lookup ends right here — this is why a DNS change can appear “instant” on one machine and broken everywhere else.
• Recursive resolver: usually your ISP’s DNS or a public service such as 8.8.8.8 or 1.1.1.1. It answers from its cache when it can, and otherwise walks the hierarchy for you.
• Root servers: they do not know the answer, but they know which servers handle the TLD (such as .com or .ir).
• TLD servers: they return the NS records — the nameservers responsible for the domain.
• Authoritative nameserver: the final authority for the zone; it returns the actual A, AAAA, MX or TXT record.

You can watch each step yourself:

dig example.com A +short
dig example.com NS
dig @8.8.8.8 example.com A

Why the path matters when something breaks

If a site does not open but a direct connection to the IP works, the fault is almost always somewhere in this chain: a wrong A record, stale NS records left over from a hosting migration, or a high TTL keeping an old answer alive in caches around the world. Because every resolver along the way caches answers, two users can see two different versions of your DNS at the same time — one pointing at the old server, one at the new.

That asymmetry is the key to DNS troubleshooting: asking the authoritative nameserver directly shows what the zone is supposed to say, while querying public resolvers shows what the world currently sees. The gap between the two is propagation, and it closes only as caches expire.

When you change a record — especially MX or NS, where mistakes are expensive — check the result from several geographic vantage points rather than trusting a single lookup from your own machine. The WhoDNS DNS check runs the full record set (A, AAAA, MX, NS, TXT and more) for any domain and shows you exactly what the public internet resolves, which is the fastest way to separate a zone problem from a caching problem.

🔗 Related tool: Run a full DNS check on any domain