MEPX
Chapter 3 of 10All chapters
  1. 1Layers
  2. 2Addresses
  3. 3DNS
  4. 4TCP and UDP
  5. 5Routing
  6. 6HTTP
  7. 7Wifi and local networks
  8. 8Latency and bandwidth
  9. 9Diagnosing problems
  10. 10How the internet fits together

Chapter 3 of 10

DNS

Names into addresses.

The lookup

DNS translates a domain name into an IP address, asking a chain of servers until one answers. Results are cached at every step, which is why changes take time to spread.

  • TTL sets how long an answer may be cached.
  • An A record points at an address; CNAME points at another name.

Why it looks like an outage

If DNS fails, everything appears down even though the servers are healthy. It is the first thing to check when a site is unreachable by name but fine by address.