DNS propagation is the period after you change a DNS setting when some people see the new version and others still see the old one. Nothing is actually being pushed around the world. Resolvers everywhere are holding cached copies of the old answer, and each one only asks again when its copy expires.
Why the old answer lingers
Every record carries a TTL, a number of seconds that resolvers may reuse the answer before checking again. If your A record had a TTL of 3600, a resolver that looked it up just before your edit can keep serving the old IP for up to an hour. Different resolvers fetched at different times, so they expire at different times, which is why your phone and your laptop can disagree.
Nameserver changes are usually slower than record changes, because the delegation for a domain is cached from the TLD's servers. For .com and .net names that delegation carries a two-day TTL, which is where the familiar "allow up to 48 hours" advice comes from. Browsers and operating systems also keep short caches of their own.
Example: moving a site with almost no overlap
A tutoring business plans to move hosts on Saturday. On Thursday, the owner lowers the TTL on the A and www records from 3600 to 300 seconds. By Saturday, every resolver has picked up the shorter TTL. She changes the IP, and within about five minutes nearly every visitor reaches the new host. Once things look stable, she raises the TTL again. Keeping the old host running for a couple of days covers any stragglers.
When propagation affects you
Propagation only matters at the moment you change something: launching on a new host, switching email providers, or moving DNS. Day to day, you can ignore it. Online checkers that query resolvers in many countries are handy for watching a change roll out, but they show a sample, not every resolver on earth.
What people get wrong about propagation
- Blaming propagation for a typo. If a public resolver like 1.1.1.1 still returns the old value well after the TTL has passed, recheck the record, and check that you edited it at the provider your nameservers point to.
- Lowering the TTL at the same time as the change. Resolvers are still holding the old, long TTL. Lower it a day ahead.
- Shutting down the old server or mailbox immediately. Some visitors and some mail will still arrive there for a while.
- Repeatedly editing while waiting. Resolvers that refresh at different moments pick up different versions, so you can no longer tell which edit a visitor is seeing.
Related terms
- TTL: the timer that sets how long propagation takes.
- DNS: the cached system being updated.
- Nameservers: changes to these take longest.
- Browser cache: another local cache that can show stale content.