Your firewall is largely gone the moment you plug into a LAN (subnet). Most of today's consumer firewalls are provided as an additional functionality by the NAT in your router/access point/switch/dhcp server.
That's also why people in an subnet can reach each others ports without getting blocked. There is this thing called the windows firewall but as far as I can tell that won't block much (and most people will just click OK on the popup anyways). Also that would be assuming that people actually use windows. (you said you want to make it linux compatible)