Art-Net numbering, my old nemesis. Think of the number as a house number on the street. It's how you let other applications know "I live in the fourth house on this street". The problem is that each developer decided to come up with their own way to count those houses.
The "official" Art-Net 2 spec divides houses into Universes and Subnets, each with 16 possible options, counted from 0. So, the first house is on Subnet 0, Universe 0. The next house is on Subnet 0, Universe 1. Etc etc. After you reach house number 16 at Subnet 0, Universe 15, the next 16 houses are on Subnet 1, Universe 0 through 15. Etc etc. All the way up to the 256th house at Subnet 15, Universe 15.
This is the official way of counting in Art-Net. It's what we do and what Artnetominator does. Madmapper does things slightly different, in that they don't use Subnets, and just count Universes from 0 to 255. GrandMa also doesn't use Subnets, but they start counting at 1. Looks like your app does things like GMA.
Basically, the rule of thumb is that your first house will be at the lowest possible option. In Resolume, that's 0:0. In Madmapper, it's 0. In GMA and your app, it's 1. The next house will be at 1 higher than that. It's good to remember that we're still talking about the same house, the first house on the street. Your app and Resolume just number it differently.
That's the theory. To really troubleshoot connections, it's best to use Artnetominator. It's a very handy tool that let's you see what Art-Net traffic there is on your network, and what Subnet and Universe are currently being used.
It's very useful to have this info, because there's a lot of things that come into play, aside from counting. For instance, sending Art-Net messages to 127.0.0.1 is officially outside of the Art-Net spec, so some purist applications won't let you do that. Also, some apps (like us in the past) will open the Art-Net port "exclusively", which means that one app can send and receive fine on it, but other apps on the same machine can't.
