the DNS time-to-live
Every DNS record carries an expiry timer called its time-to-live, or TTL — a number of seconds telling anyone who caches it how long they are allowed to keep using the copy before they must look it up fresh. Think of a sticky note with best before written on it: while the time is good, reuse the note; once it expires, throw it away and ask again. The TTL is how the owner of a name controls the trade-off between speed and freshness.
Concretely, the TTL is set per record by the zone's administrator, and it is a number of seconds — values like 300 (5 minutes), 3600 (1 hour), or 86400 (1 day) are common. When a resolver caches the record, it counts down that TTL; while it is still positive, the resolver answers from cache, and when it hits zero the resolver discards the entry and will fetch a fresh copy next time. Note the TTL does not mean the record changes every so often — it only governs how long a cached copy may be trusted.
Why it matters: TTL is the dial behind DNS propagation. A long TTL means lookups are fast and authoritative servers see little load, but a change takes that long to spread because old copies linger. A short TTL makes changes propagate quickly but costs more lookups and load. The practical playbook before a planned migration is to lower the TTL well in advance (say to 60 seconds), make the change, then raise it again once everything is stable. A common misconception is that you can force the whole world to update instantly — you cannot reliably; caches honor the TTL that was in effect when they fetched the record.
An A record set with TTL 3600 is fetched at 12:00. A resolver may serve that cached address until 13:00 without rechecking. If the owner changes the IP at 12:10, resolvers that cached it at 12:00 keep handing out the old IP until their hour is up.
TTL is the lifetime of a cached copy, set by the record's owner.
Lowering a TTL only affects copies fetched after the change; copies already cached run out on their old, longer TTL. So you must lower the TTL and wait for the old one to expire before the short window takes effect.