Lesson 2 of 412 min

DNS (Domain Name System) — the phone book of the internet

A/AAAA, CNAME (Canonical Name record), MX (Mail Exchange record), TXT, NS. TTL (Time To Live). Recursive vs authoritative. Why nslookup beats opinion.

Network switch with a bundle of ethernet cables — Photo by Brett Sayles on Pexels

DNS turns names (mail.contoso.com) into IPs. Record types: A = IPv4, AAAA = IPv6, CNAME = alias, MX = mail server, TXT = arbitrary (SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), domain verification), NS = which DNS server is authoritative.

TTL (Time To Live) controls how long records are cached. Low TTL before a migration; raise it after.

Diagram · Small-office network

When something 'just stopped working', `nslookup` or `dig` the record from the user's machine — and from outside the network.

Key takeaways

▸TXT records hold most email security
▸TTL is your migration lever
▸Always test DNS from inside AND outside

Areas to show initiative · +15 XP each

Document a client's complete DNS zone in their docsLower TTL 24h before a planned IP (Internet Protocol) change

Next: DHCP and just-enough subnetting →