The codes for IFR flight plans are generated by computers in the individual
ARTCCs.. There are only 4096 possible codes, and not all of them can be
used. The military has control of a large portion. The same code may be
used at the same time in different parts of the US.
Beacon (transponder) codes are assigned in accordance with FAA Order
7110.66, titled National Beacon Code Allocation Plan (NBCAP).
Each ARTCC is assigned code blocks for their external and internal
departures.
An external departure is one that leaves the originating center's
airspace,
An internal departure is one that begins and ends within the same
center.
Blocks of transponder codes for external departures are used by only one
ARTCC, but the same
blocks for internal departures can be used by several ARTCCs that do not
have a common boundary. For example, the 6500 code block may be used by
Chicago Center for external departures, no other ARTCC uses that block.
Chicago ARTCC uses the 4300 code block for internal departures, but none of
the four centers that border Chicago may use that block of codes. The 4300
block is used for internal departures by New York, Salt Lake City,
Jacksonville, Memphis and Albuquerque Centers.
Codes 0100 to 0477 (or portions thereof) are assigned for internal use by
terminal facilities.
These are for flights that are not entered in the center's flight data
processing computer; VFR operations and local IFR flights. Blocks of codes
are assigned so that adjacent terminal facilities do not use the same sets.
Additional blocks of codes may be assigned to busy terminal facilities.