if u're using dhcp, it will recycle IP. usually for filtering, we would use MAC rather than IP cos MAC is tied to the device.
there's 2 ways of filtering depend on how ur router works:
blacklist: blocks out those in ur list
whitelist: only those in ur list can use
i'm also using a 3G modem with MMORPG, frankly playing MMORPG game on 3G wont lag much on the internet. it's when u play FB, the initial loading will kill almost all others. also 1Mbps 3G is only sufficient for 1 user. more than 1 user and u'll find when the other is downloading u'll lag. also 3G is not built for speed, it fluctuates, even with 1 person using, i find it's lagging at times n i have to reconnect.