How DNS Translates Domain Names to IP Addresses: A Step‑by‑Step Guide

Why DNS Matters

During a recent interview, a candidate was asked about the DNS request process and struggled to answer, prompting a deeper look at the definitions and principles behind DNS.

DNS Meaning and Structure

DNS (Domain Name System) is an Internet service that maps human‑readable domain names (e.g., www.51cto.com) to IP addresses. It operates as a distributed database organized in three layers:

Root DNS servers store the IP addresses of top‑level domain (TLD) servers.

TLD servers handle three categories of domains: gTLD (generic, e.g., .com, .net), ccTLD (country code, e.g., .cn, .jp), and New gTLD (e.g., .xyz, .top).

Local DNS servers (often provided by ISPs) cache query results and act as the first resolver for end users.

The hierarchy can be visualized as:

Figure 1: DNS hierarchical structure.

DNS Resolution Process

The resolution of a domain name to an IP address involves nine steps, illustrated below:

Figure 2: Full DNS request flow.

The user enters a URL in the browser. The browser first checks its own cache for a matching IP.

If not cached, the operating system’s hosts file (e.g., C:\Windows\System32\drivers\etc\hosts on Windows or /etc/hosts on Linux) is consulted.

The query is sent to the Local DNS server provided by the ISP or campus network, which often satisfies the request from its cache (about 80% of cases).

If the Local DNS lacks the answer, it contacts a Root DNS server to obtain the address of the appropriate TLD server.

The TLD server (gTLD, ccTLD, or New gTLD) returns the authoritative Name Server for the domain.

The Name Server provides the domain’s A (address) record or CNAME (alias) record and sets a TTL (Time‑To‑Live) value.

If an A record is returned, the browser can contact the target IP; many large sites instead return a CNAME that points to a Global Traffic Manager (GTM).

The GTM performs global load balancing, directing the request to the nearest or healthiest server, often behind a CDN.

Static resources (HTML, CSS, JS, images) are fetched from the CDN cache, while dynamic content (e.g., product or order data) is retrieved from the application server behind a load balancer.

Conclusion

DNS servers translate URLs to IP addresses through a three‑tier hierarchy: Root, TLD, and Local DNS. The full resolution path typically traverses the browser cache, hosts file, Local DNS, Root DNS, TLD DNS, authoritative Name Server, GTM, CDN, and finally the application server, encompassing nine distinct steps.