You switched hosting, pointed the domain at the new server and... the site still loads the old one. Or it loads the new one on your phone and the old one on your computer. That's the famous "DNS propagation". The name is a bit misleading, and understanding what really happens helps you plan any change without surprises.
What really happens
Nothing "spreads" across the internet. When someone visits your domain, their computer asks a DNS resolver for the address, usually the one run by their internet provider. That resolver fetches the answer from your domain's servers and caches it for a while, so it doesn't have to ask again on every visit.
That time is the TTL (time to live), which you set on each record. Until the TTL expires, the resolver keeps handing out the old answer, even if you've already changed the record. "Propagation" is just thousands of caches expiring, each on its own schedule.
How long it takes, in practice
- Changing an A, CNAME, MX or TXT record: the maximum time is the TTL the record had before the change. With a TTL of 300 seconds, almost everyone sees the new value within 5 minutes. With 3600, within 1 hour. With 86400, within 24 hours.
- Changing name servers (NS) at the registrar: usually takes from a few hours to 1 or 2 days, because the TLD's NS records have long TTLs. Registries like Verisign publish the change quickly, but caches still need to expire.
- New domain: there's no old cache, so it usually works within minutes of being published.
The "up to 48 hours" you see repeated everywhere is an old safety margin. With a low TTL, the change is almost immediate.
Why some people see the new site and others the old one
Each resolver cached the answer at a different moment, so each one expires at a different moment. On top of that, your computer and your browser have their own caches. That's why it's common for a phone on mobile data to show the new site while a computer on Wi-Fi still shows the old one.
How to track it
Use the DNS propagation checker: it asks 15 public resolvers in several countries for the same record and shows how many already see the new value. To see all of a domain's records at once, use the DNS lookup.
To see the new value on your computer before the cache expires:
- Windows: open Command Prompt and run
ipconfig /flushdns. - macOS: in Terminal,
sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder. - Browser: test in a private/incognito window, or restart the browser.
How to migrate with zero downtime
- One day before, lower the TTL of the records you'll change to 300 seconds. Wait for the old TTL to expire.
- Get the new server ready and tested before touching DNS.
- Change the records. With a low TTL, the switch happens in minutes.
- Keep the old server running for at least a day, for anyone who still has the old answer cached.
- Once everything is stable, raise the TTL back to a higher value, such as 3600, to reduce lookups.
For email, take the same care with MX records: keep the old mailbox receiving until propagation is complete, so you don't lose messages.