What is DNS? A beginner's guide to the internet's phone book

DNS is the system that turns modusdom.com into the actual computer your browser connects to. It's the most critical piece of internet infrastructure most people never think about — until it breaks, at which point it looks like the entire internet has crashed.

The metaphor that explains everything

DNS is the phone book of the internet.

Every computer on the internet has a numerical IP address — 192.0.2.1, 2606:4700::6810:84e5. Humans can't remember those. We can remember modusdom.com. DNS is the lookup that connects the two.

When you type modusdom.com into your browser:

1. Your computer asks a "recursive resolver" (usually your ISP's, or Cloudflare's 1.1.1.1, or Google's 8.8.8.8): "What's the IP for modusdom.com?"
2. The resolver doesn't know offhand, so it asks the root nameservers (13 globally distributed clusters): "Who handles .com?"
3. Root replies: "Verisign does. Here are their nameservers."
4. The resolver asks Verisign's nameservers: "Who handles modusdom.com?"
5. Verisign replies: "Modusdom's own nameservers: ns1.modusdom.com and ns2.modusdom.com."
6. The resolver asks ns1.modusdom.com: "What's the IP for modusdom.com?"
7. ns1.modusdom.com finally answers with the actual IP address.
8. The resolver caches the answer (and tells your computer), so the next person asking gets the answer instantly.

The whole roundtrip typically takes 20–100 milliseconds. After that, it's cached for hours.

Authoritative vs. recursive nameservers

Two flavors:

• Authoritative nameservers hold the actual answer for a specific domain. ns1.modusdom.com is authoritative for the modusdom.com zone. We control what it says.
• Recursive resolvers just ask authoritative nameservers and cache the answers. Your home router runs a tiny one. Cloudflare's 1.1.1.1 and Google's 8.8.8.8 are global ones serving billions of queries a day.

DNS records — what actually lives in a zone

The data attached to a domain is a list of resource records. The common ones:

• A — "this name points to this IPv4 address." e.g., modusdom.com A 192.0.2.42.
• AAAA — same but for IPv6.
• CNAME — "this name is an alias for that other name." Used to point www.yourdomain.com at a cloud host like yourdomain.netlify.app.
• MX — "deliver email for this domain to this mail server." Multiple MX records can be listed with priority numbers.
• TXT — free-form text. Used for SPF, DKIM, DMARC, domain verification at services like Google or Stripe.
• NS — "the authoritative nameservers for this domain are these." This is set at the registry level, not in the zone itself.
• SRV — "service X is at this hostname and port." Used by SIP, XMPP, Microsoft 365 autodiscover.
• CAA — "only these certificate authorities can issue SSL for this domain." Anti-mis-issuance.

TTLs and why DNS changes feel slow

Every DNS record has a TTL (time to live) — how long resolvers should cache the answer before re-asking. Typical values: 1 hour (3600 seconds), 24 hours, sometimes a week.

When you change a DNS record, existing caches don't refresh until their TTL expires. That's why DNS changes feel slow: you updated it, but every recursive resolver in the world is still serving the old answer until their cache times out.

Pro tip: lower your TTL to 5 minutes before you plan to make changes, wait until the previous TTL has expired so the new low one propagates, then make your change. Bump it back up to 1 hour afterward to reduce resolver load.

Common DNS failures

• "Domain doesn't resolve." Usually means your nameserver records (NS) at the registry point at nameservers that don't have a zone for your domain. Check the NS at the registry matches where your DNS zone actually lives.
• "Website works but email doesn't." A record is fine; MX records missing or wrong. MX records guide here.
• "Site worked yesterday, broken today." Usually a TTL just expired and a record you forgot you had is now serving an outdated value.
• "It works for me, not for them." DNS propagation in progress — some resolvers have the new answer cached, others still have the old. Wait for TTL to clear globally (a few hours typically).

Where Modusdom fits

If you use Modusdom's nameservers (ns1.modusdom.com, ns2.modusdom.com), you manage all your DNS records from your account dashboard. If you'd rather use Cloudflare, AWS Route 53, or someone else, switch your nameservers on the domain page — we just hand off control.

For email: we configure SPF, DKIM, and DMARC automatically when you provision a mailbox through us. See the deliverability deep-dive for what each of those records does.