APN
APN (Access Point Name) is the configuration string your phone uses to connect to the carrier's data network. Wrong APN = no mobile data. Usually auto-configured at SIM/eSIM activation.
APN stands for Access Point Name — the configuration string your phone uses to reach the carrier's mobile data network. Without a correct APN, your phone can connect to the cell tower for voice and SMS but can't actually use mobile data. Most users never see APN settings; carriers push them automatically when you activate a SIM or eSIM. APN visibility comes up when something has gone wrong.
When APN problems appear
- After porting your number to a new carrier and seeing "no service" or "data not working" while calls/SMS work fine. The phone has the new SIM but is still using the old carrier's APN settings.
- BYOD activation on certain prepaid carriers where the carrier doesn't push a configuration profile. You manually set APN to make data work.
- Switching between dual-eSIM lines when one of them needs different APN values.
- International travel with a local SIM that didn't auto-configure on your phone model.
How to fix
iPhone: Settings → General → Transfer or Reset → Reset Network Settings. This wipes saved Wi-Fi passwords too, but resets APN so the new carrier can push the right one. Or install the carrier's configuration profile if they offer one (carrier sends a download link via the welcome SMS).
Android: Settings → Mobile network → Access Point Names. Tap the menu and "Reset to defaults," or manually create a new APN with values from the carrier's support page.
What's actually in an APN config
An APN entry has a name (cosmetic), an APN string (the host the phone routes data through, e.g., "fast.t-mobile.com"), an authentication type, sometimes a username/password, and a list of supported services (default, mms, supl). Carriers' APN values are publicly documented; you can usually find the right ones by searching "[carrier name] APN settings" if auto-configuration fails.