Obsonet

How about euhm... requiring an extra/not used networkcard for emulation of Obsonet on PC? You can get a PC networkcard for a few bucks these days. (Not that Obsonet is that expensive...)

Snout: That might work if the OS gives an application rights to all ports, which is true on Windows as far as I know. However, on Unix-like systems, which come from a multi-user background, only the superuser (root) has access to ports below 1024.

I'm also not sure if programs that listen to a port on all interfaces claim that port exclusively on all interfaces, or whether they only connect as a kind of fallback if no program claimed that port on a specific interface. If the former is the case, it will be almost impossible to keep all ports available for the emulator on a specific network interface.

The advantage of your option is that it's easier to implement than the others. But it has some portability problems and installing an extra network card + cable is rather large effort for the user. Also this assumes the user already has an in-home network (small LAN), while some people just have 1 PC connected directly to a cable/ADSL modem.

So it might be a useful step for testing the hardware emulation, but I don't think it's the definitive solution for MSX network emulation.

It's even worse, access on all ports is not good enough to run emulation of a network device over. You need raw packet access. Also only available as root under unix like systems.
Not necessarily, they could just break right through the OSI layer model, decode TCP / UDP packets and only support that as a protocol.

~Grauw

It would be cool if there would be a IRC client for the MSX

Of course it is emulation . Just not complete emulation, due to the limits imposed by the host OS. Still better than no emulation at all though.