Every APRS callsign you see on a map is really two things: a callsign, and a number after a dash. W1ABC-9 is a different station from W1ABC-10, even though the same operator holds the same licence for both. That number is the SSID, and this page is the lookup table people are usually after, plus the parts of the story that the table alone does not tell.
If you want the bit-level detail of how the SSID is encoded inside an AX.25 address, that lives in AX.25 Protocol Explained. This page is the practical version: which number to pick, and what other operators will assume when they see yours.
SSID stands for Secondary Station Identifier. It is a 4-bit field, so it holds the values 0 to 15, and it is written after the callsign with a dash. One licence, sixteen possible simultaneous stations.
Nothing on the APRS network enforces what each number means. The list below is convention, published in APRS Spec 1.2 chapter 21 and in Bob Bruninga's SSIDs.txt, and followed closely enough that other operators read meaning into it.
| SSID | Conventional meaning | Typical real-world station |
|---|---|---|
-0 (or none) |
Primary fixed station | Your home station, usually message-capable |
-1 |
Generic additional station | A second digi, a spare box, an experiment |
-2 |
Digipeater | A fill-in or wide-area digipeater |
-3 |
Generic additional station | Often a second digipeater |
-4 |
Generic additional station | Anything that does not fit elsewhere |
-5 |
Other networks | D-STAR, DMR gateways, phones, Android clients |
-6 |
Special activity | Satellite work, camping, 6 m operation |
-7 |
Handheld | HTs and anything human-portable |
-8 |
Boats, RVs, second mobile | Marine and motorhome stations |
-9 |
Primary mobile | The classic car, usually message-capable |
-10 |
Internet-side station | iGates, EchoLink, Winlink |
-11 |
Balloons, aircraft, spacecraft | High-altitude balloon payloads |
-12 |
One-way trackers | APRStt, DTMF, RFID, beacon-only devices |
-13 |
Weather station | A WX box feeding temperature and wind |
-14 |
Truckers | Full-time drivers, long-haul rigs |
-15 |
Generic additional station | The other catch-all slot |
Two entries in that table trip people up more than the rest, so they are worth saying plainly.
-0 and no SSID are the same address. W1ABC and W1ABC-0 are byte-for-byte identical once encoded into an AX.25 address field. Software that displays them differently is being cosmetic, not technical.
-1 and -2 are not interchangeable. The digipeater convention is -2, not -1. -1 drifted into general use as a spare, and you will find plenty of digipeaters running on -1 in the wild, but if you are standing up a new one, -2 is the number that will not confuse anybody.
Most operators only ever need three or four of these.
-9.-7.-10.-2.-13.-11.If you run several of these at once, they are separate stations on the map, with separate tracks, separate symbols and separate message queues. That is the point of the field.
Nothing breaks. No server rejects your packets, no digipeater refuses to repeat them, and your position still appears on the live map. The SSID is metadata, not access control.
What you lose is legibility. Other operators scan a station list and infer from the SSID whether you are a car worth messaging, an iGate worth thanking, or a one-way tracker that cannot reply. A weather station on -9 reads as a car that never moves, and a mobile on -13 reads as a weather box that inexplicably drives around. Neither is a fault. Both waste somebody's attention for a moment.
The one case where an SSID choice has a real technical consequence is a one-way tracker on -12. Some clients and net-control tools use the SSID as a hint about whether a station can receive messages, and will not attempt to send to a -12. If your device really can receive, do not label it -12.
A frequent source of confusion, and an easy one to settle: your APRS-IS passcode is computed from the base callsign only. The SSID is stripped before the calculation.
That means W1ABC, W1ABC-5 and W1ABC-10 all share one passcode. You do not need a new one for each station you bring online. If you need yours, the passcode generator computes it in the browser and never sends the callsign anywhere.
The 0-15 range is what fits in an AX.25 address field. Stations that connect straight to APRS-IS over the Internet and never key a radio are not bound by that field, and some of them carry identifiers that could never survive an RF hop: SSIDs with letters (OE6AVD-D, SR3DOS-B), callsigns longer than six characters, lower-case text, or object names containing dots and slashes (444.575IA, F1ZQT/B).
These are legitimate APRS-IS participants and they show up on maps, including ours. They are simply Internet-side objects rather than AX.25 stations. If you are writing software that parses APRS callsigns, do not assume the SSID is numeric — that assumption is one of the more common ways a parser quietly loses stations.
If you want to see what a given callsign-SSID is doing right now — its symbol, its path, its last packet, which radio it appears to be — open its station page directly. Every callsign on the network has one, for example /station/W1ABC-9 style URLs, reachable from the station directory.
To read the raw frame behind a beacon and see the SSID in context with the digipeater path, paste it into the APRS Packet Decoder.
-10 belongs and why.