DNS TTL
Problem
Caching DNS makes it fast.
But if example.com changes IP, cached clients may continue using the old address forever.
Solution — TTL
DNS records specify how long caches may reuse an answer.
example.com
203.0.113.10
TTL = 300
After that, the resolver asks again.
Core idea: DNS TTL balances caching efficiency against how quickly DNS changes propagate.