From what I understand is that there are 3 different networks. I have a very low understanding of what panels are comparable with what network. ES net (4100es, 4010es, 4017es, 4007es), the 4120 network (4100es, 4017es?, 4007es, 4100U, 4010, 4008?, 4006?, 4120, 4020?, 4100+?, 4002) and the 2120 (4020?, 4100?, 4120?)
2120 is not a network type that exists.
In regards to the other two – mostly correct.
ES Net networks are exclusive to the 4100ES, 4010ES, 4017ES and 4007ES, and new gen NSI panels, as well as 4190 TrueSite Workstations/Incident Commanders. These networks use different hardware than the 4120 network does, and support a few more features (such as connectivity over Ethernet). They also still, of course, support fiber optic (multiple types) and wired media.
4120 (now legacy, they are officially ‘discontinued’ by Simplex) networks officially support 4100ES, 4100U, 4100+, 4100 classic, 4010ES, 4007ES, 4010 classic, 4020, 4002, 2120, NPU, 4190 TrueSite Workstation/Incident Commanders (and IMS, aka the older TSW), 2500 NDU, and old gen NSI panels.
Older systems (such as the 4100+/classic or 4020) must be updated to a newer firmware revision in order to play nicely with the rest of the network.
There are also several NDU types compatible with each network, such as the ES Net NDU and 4120 NDU, respectively.