1200 is VFR, which generally means “I’m flying visually and don’t need ATC help”. If you’re squawked 1200 you show up as VFR on their screen, but you still show up. That doesn’t mean null. 7500, 7600, and 7700 are used for various emergency purposes, with 7700 being the most common, and almost always accompanying a mayday or panpan. Those are transponder codes, not radio channels. It’s a code returned by your equipment when the transponder is painted with interrogative radar.

0000 is closer to “null”, but still isn’t quite. 1000 also has some “null” like properties when it comes to ADS-B. Note that what I’m saying is North American centric and not necessarily ICAO nor other areas, which can differ somewhat.

never ceases to amaze: https://en.wikipedia.org/wiki/List_of_transponder_codes

1200 is VFR. That doesn’t mean null. 7500, 7600, and 7700 are used for various emergency purposes. These are transponder codes, not radio channels. It’s a code returned when the transponder is painted with interrogative radar.

This being Hacker News: The reason the digit 7 seems important here is that these are actually Octal. Under the hood this is a digital system, but the user interface is four octal digits ie 0000 to 7777 is a 12-bit value.

Most modern aircraft are capable of providing a lot more data over their radio transponder, including a system unique identifier, but it turns out that "squawking" a four digit code is a useful amount of discretion to give humans. If you could do it over maybe a decimal code would have been better, but too late now.

when you look at all the reserved blocks, that's not a lot of usable address space. the internet says there are roughly 5k commercial aircraft in the air above the us during peak times. i get that these are regionally allocated, but still pretty tight i must imagine.

do modern mode 3 transponders also include tail number or some other unique identifier in a sideband?

Yes, the squawk code is not itself necessarily used to identify individual airplanes and distinguish them from one another and transponders now transmit significant amounts of data on top of the squawk code.

https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillan...

(I don't know how much squawk codes are, or were, used to specifically identify aircraft in the past, but I believe that currently it's not unusual that many aircraft in the same region would be squawking the same number, which would not confuse ATC because of all the other transponder data that's available.)

This depends strongly on the region. The US still has a very old system in use for the center controllers which are the big regions. That system wants discrete codes for each aircraft in each area, so they make flights change code if there is overlap while flying into the next region.

In Europe some areas (but not all) have switched to using the Mode-s/ADS-B identification (which is a 24 bits unique code not configurable by the pilot but fixed to the aircraft) in their systems and setting the traditional transponder code to 1000 for all of those aircraft.

In the long term expect everyone to adopt that approach, but things in aviation move very slowly so it will be many many years before we're there.