Guide
Callsigns, registrations and hex codes
Every aircraft has three kinds of identifier: a callsign for the current flight (such as DAL1520), a registration painted on the airframe (such as N301DN) and a permanent 24-bit ICAO address broadcast by its transponder (such as A39A8C). Trackers use all three, but the hex address is the one that never lies about which airframe it is.
Callsigns and flight numbers
A flight number is what you see on your ticket: DL 1520, with Delta's two-letter IATA code. A callsign is what the crew uses with air traffic control and what the aircraft broadcasts: DAL1520, using Delta's three-letter ICAO designator. On the radio it is spoken with the airline's telephony name: "Delta fifteen twenty."
The two often match, but not always. Airlines sometimes use alphanumeric callsigns, such as UAL12AB, to avoid confusion when two flights with similar numbers are on the same frequency. Codeshare flights broadcast only the operating airline's callsign, so a United Express flight run by SkyWest shows as SKW plus a number. Our airlines and operators guide lists the common prefixes.
Private aircraft usually use their registration as the callsign, such as N123AB. Military flights use mission callsigns like REACH (Air Mobility Command) followed by digits. Trackers display whatever the crew types in, so typos and blanks happen.
Registrations
A registration is a national identifier, like a licence plate. Each country has a prefix: N for the United States, C for Canada, XA/XB/XC for Mexico, G for the United Kingdom, D for Germany. US registrations are called N-numbers, and can be N followed by up to five characters (for example N1, N12345 or N123AB). The FAA aircraft registry is public.
Hex codes: the ICAO 24-bit address
Every Mode S transponder and ADS-B unit is programmed with a 24-bit address, usually written as six hexadecimal characters. ICAO allocates blocks to countries; the US block covers addresses beginning with A (A00000 to AFFFFF).
The N-number connection
For US civil aircraft, the FAA maps N-numbers to addresses with a fixed algorithm: the range A00001 to ADF7C7 covers every possible N-number, from N1 through N99999 including those ending in letters, in a fixed order. A00001 is N1, A00002 is N1A, and so on. That means a US civil aircraft's hex address can be decoded into its registration without a database, and re-registering the aircraft with a new N-number changes its address.
Addresses above ADF7C7 in the US block are used for other purposes, including many US government and military aircraft, which commonly appear with addresses starting AE. See why military aircraft show or don't.
Why the hex code is the best identifier
- It is broadcast in every message, even when no callsign is set.
- It belongs to one transponder in one airframe and doesn't change from flight to flight.
- It is how airplanes.live links work:
globe.airplanes.live/?icao=a39a8c.
To check what an address is doing right now, use our aircraft lookup.
Privacy programs: LADD and PIA
The FAA runs two programs for aircraft owners who want less visibility:
- LADD (Limiting Aircraft Data Displayed) asks providers who receive FAA flight data to hide or limit display of an aircraft. It applies to FAA-sourced data feeds.
- PIA (Privacy ICAO Address) lets eligible owners use a temporary 24-bit address not publicly linked to their registration, and a matching callsign.
Community networks like airplanes.live receive signals directly over the air rather than from the FAA, and show what their receivers hear. An aircraft using PIA appears with an address that does not decode to its N-number and often with no type or registration. heywhatsflying.com shows aggregates and notable aircraft, not tracking of individuals.
Other countries' address blocks
Within our US snapshot you will regularly see foreign addresses too. Canadian aircraft use addresses starting with C0 to C3, and Mexican aircraft use the 0D block. Long-haul European arrivals carry addresses from their own national blocks, for example 40 to 43 for the United Kingdom and 3C to 3F for Germany. The address tells you the country of registration, not the airline or where the flight began.
Putting it together
| Identifier | Example | Changes? | Source |
|---|---|---|---|
| Flight number | DL 1520 | Per scheduled flight | Airline schedule |
| Callsign | DAL1520 | Per flight | Typed by crew, broadcast |
| Registration | N301DN | Rarely | National registry |
| Hex address | A39A8C | Only with re-registration | Transponder, broadcast |
The example values are illustrative. See also how to read flight data and the glossary.
Questions
What is the difference between a flight number and a callsign?
A flight number uses the airline's two-letter IATA code, such as DL 1520. The callsign broadcast to air traffic control uses the three-letter ICAO code, such as DAL1520, and may differ from the flight number.
Can you find an N-number from a hex code?
For US civil aircraft, yes. The FAA maps the range A00001 to ADF7C7 to every possible N-number with a fixed algorithm, so the hex address decodes to the registration.