Smash isn't supported by LAN play by itself. Smash uses local wireless, not LAN, to communicate locally so you would need the idn_mitm.kip enabled to suppress the local wireless signal and have it use LAN instead. This means both parties need CFW since you can't load that kip without it (SX OS comes with it built-in but you would need to pay for the license while the other CFW are completely free). Once everyone is using CFW and idn_mitm.kip, you can use the local wireless feature of Smash to play with anyone across the globe, after you configured the proper IP addresses and other settings required for LAN play.