gigabit netconnection koi v ISP provide nahin karda, only very few wholesale carrier(which provide bandwidth to carriers only) provide gigabit and 10 gig bandwidhths, few of them are
Cogent
Level3
Savvis
it may be the case that your modem and NIC in your pc are both set to negotiate at auto speed and auto duplex settings .generally modem's lan port is hardcoded to 100/full . but if not and both sides are gig capable they can negotiate at 1000 or 1G full