DNS — names to addresses
Key takeaways DNS (the Domain Name System) is the internet’s phone book: it turns names people remember into the IP addresses machines actually use. Your resolver does the lookup — checking its cache, then asking up the hierarchy until it finds the answer. The answers come as records: an A record maps a name to an IPv4 address, AAAA to IPv6, CNAME aliases one name to another, MX points to mail servers, and TXT holds text used for verification. To keep it fast, results are cached for a TTL before they’re looked up again.
Every time you type a name and a page loads, something quietly turned that name into a number first. That something is DNS, and it runs so smoothly you rarely notice — until it breaks. Understanding it makes a whole class of “the site is down” mysteries obvious.
The problem DNS solves
People remember names. example.com is easy; the machine behind it lives at a
numeric IP address like 93.184.216.34 (or an
even longer IPv6 address), which nobody wants to memorise. But the network itself
only knows how to deliver data to addresses, not names.
DNS bridges that gap. It’s a giant, distributed lookup system whose one job is to answer the question “what address is this name?” You work with the friendly name; DNS quietly supplies the number underneath so your traffic reaches the right place.
How a lookup works
When your device needs the address for a name, it doesn’t search the whole internet itself — it asks a resolver (usually run by your ISP or a public service) to do the legwork.
The resolver works in steps:
- Check the cache. If it looked this name up recently, it already has the answer and returns it immediately.
- Ask the root. If not, it starts at the top — a root server — which
doesn’t know the address but knows who handles the top-level domain (the
.com,.org, or.netpart). - Ask the top-level domain. That server points to the authoritative nameserver for the specific domain — the one that actually holds its records.
- Ask the authoritative nameserver. It returns the real answer, and the resolver passes it back to your device.
It sounds like a lot of hops, but it happens in milliseconds, and thanks to caching most lookups never travel the full path.
Why caching and TTL matter
Every answer comes with a TTL — a time to live, in seconds — that says how long it may be reused before it must be looked up again. While a record is cached, resolvers hand out the stored answer instead of walking the hierarchy, which is what keeps DNS fast.
The trade-off shows up when a record changes: until the old TTL expires, resolvers around the world keep serving the stale answer. That’s why a DNS change can take a while to “take effect” everywhere. Lowering the TTL ahead of a planned change lets the new answer spread more quickly.
Common record types
A domain’s records come in several types, each answering a different question:
| Record | Maps a name to | Used for |
|---|---|---|
| A | an IPv4 address | the everyday “where does this name live?” |
| AAAA | an IPv6 address | the same, for IPv6 |
| CNAME | another name (an alias) | pointing one name at another |
| MX | a domain’s mail servers | where email for the domain is delivered |
| TXT | arbitrary text | verification, SPF, and other proofs |
| NS | a domain’s nameservers | which servers are authoritative for it |
You’ll meet A and AAAA constantly — they’re the address records. CNAME is handy when several names should follow one target: point them all at one name and you only update that name’s address once. MX and TXT show up whenever email or domain ownership is involved.
Why it matters
DNS is what lets a name stay stable while the machine behind it changes. Move a service to a new IP address, update its A record, and everyone keeps using the same name — no one has to learn a new number.
It also underpins more than web browsing. MX records are how email finds the right servers, and TXT records are how a provider verifies you actually control a domain. So DNS quietly sits behind email delivery and domain verification too, not just websites.
And because so much depends on it, a slow or wrong DNS answer often looks like something else entirely: the connection never starts, and it feels like “the site is down” when really the name just didn’t resolve. We’ll turn that guess into a test in testing connectivity.
Seeing it yourself
You don’t have to take DNS on faith. Command-line tools like dig and nslookup
ask a resolver directly and print the records a name returns — the address, its
type, and the TTL counting down. We give them a fuller treatment in
testing connectivity; for now, just
know the lookup is something you can watch happen.
Quick check: what does DNS translate?
Recap
- DNS is the internet’s phone book — it turns names into IP addresses.
- Your resolver does the lookup: cache first, then up the hierarchy (root → top-level domain → authoritative nameserver).
- Records carry the answers: A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), TXT (verification), NS (nameservers).
- Answers are cached for a TTL, which is why changes take time to spread.
- A wrong or slow lookup often looks like “the site is down” — DNS is worth checking first.
Next up: ports & sockets