From The RadioReference Wiki
The Network Access Code or NAC is a feature of Project 25 digital radios similar to CTCSS or DCS for analog radios. That is, radios can be programmed to only break squelch when receiving the correct NAC. NACs are programmed as a 3 digit hexadecimal code that is broadcast along with the digital signal being transmitted.
Since the NAC is 3 digit hexadecimal number (12 bits), it gives 4096 possible NAC's for programming, which far exceeds its analog counterparts combined. It should be noted that 3 of the possible NACs have special meaning:
- 0x293 ($293) - the default NAC
- 0xf7e ($F7E) - a receiver set for this NAC will unsquelch on any NAC received
- 0xf7f ($F7F) - a repeater receiver set for this NAC will allow all incoming signals and the repeater transmitter will retransmit the received NAC.
Note that the above is true for commercial grade radios only; setting a scanner to NAC F7E is not the same as setting it to 'NAC Search'.
A list of NACs used by the U.S. Federal Government can be found here
Scanners that use NAC
Partial list of scanners that can filter by and/or search for NACs: