To be fair, I can't think of another OS that directly supports the things Mininet wants to do. To make it work on other platforms, they'd first have to implement those from scratch there.
Does FreeBSD nowadays provide some form of network isolation like it is available with Linux network namespaces? Nothing relevant shows up in the likely places, so I don't think it does.
Does FreeBSD nowadays provide some form of network isolation like it is available with Linux network namespaces? Nothing relevant shows up in the likely places, so I don't think it does.