Networking

DNS — Domain Name System

Almost every other page in this domain starts from an IP address already in hand. DNS is the step before that step — the lookup that turns a name a human can remember (example.com) into the numeric address IP actually needs, run automatically, invisibly, before nearly every connection on the internet.

Why names instead of just addresses

An IP address can change — a server gets migrated, a site moves to a new host, a home internet connection gets a new address from its ISP. A domain name stays stable through all of that, as long as its DNS records are updated to point at the current address. Names are also simply easier for a human to remember, type, and trust than a string of numbers.

The domain hierarchy

A domain name reads right to left as a hierarchy, most general to most specific — the opposite direction from how it's typically read aloud:

Root

The unnamed top of the entire system — represented by the invisible trailing dot in example.com..

TLD (Top-Level Domain)

.com, .org, .io — the first real level, managed by a registry for each one.

Domain

example in example.com — registered by an individual or organization.

Subdomain

www or api in www.example.com — created freely by whoever controls the domain, no registration needed.

Resolving a name, hop by hop

Looking up a name for the first time is a chain of "who do I ask next" hops, each server referring the resolver one level further down the hierarchy:

Record types

A

Maps a name to an IPv4 address.

AAAA

Maps a name to an IPv6 address.

CNAME

An alias — points one name at another name, which is then resolved in turn.

MX

Which server handles email for this domain — the record SMTP looks up before delivering a message.

TXT

Arbitrary text — commonly used for domain ownership verification and, notably, the SPF/DKIM records SMTP's own gotchas mention as the fix for outbound mail landing in spam.

NS

Which servers are authoritative for this domain — the record a TLD server hands back to point the resolver at the right place.

Caching and TTL

Every DNS record carries a TTL (Time To Live) — how many seconds a resolver is allowed to keep reusing that answer before asking again. This is why most lookups are fast: the four-hop chain above only happens once per TTL window, not on every single request. It's also why changing a DNS record isn't instant — every resolver that already cached the old answer keeps serving it until that cached copy's TTL expires, which is why DNS changes are described as "propagating" rather than taking effect immediately.

Real-world applications

• Every web and app connection — a browser, an IoT device's cloud check-in, an API client — all resolve a name before opening a single TCP or UDP connection.
• CDN geo-routing — a CDN can return a different IP address depending on where the resolver's request came from, routing users to the nearest data center without the application knowing anything changed.
• Service discovery — some systems use DNS itself (including special record types like SRV) as a lightweight way for services to find each other by name rather than hardcoded IPs.
• Load balancing via DNS — returning multiple A records for one name lets clients spread load across several servers before any smarter load balancer is even involved.

Gotchas