no it's not generally "100Mbps" at all. 100Mbps is probably the speed of the ethernet hardware which is sometimes used to link the computer and the broadband modem.
I believe the correct unit is Mbps which is megabits per second. An office ethernet connection is 10, 100 or 1000 Mbps (1000 Mbps is a gigabit connection). The internet used to be measures in kbps (kilobits per second) and a dialup was typically 56 kbps or 56 k. As broadband was introduced the connection went up to, for example, 512 kbps (0.5 Mbps) and I think the highest in the UK is around 8000 kbps or 8 Mbps (no where near 100 Mbps). I'd be happy with a 2 Mbps connection...
When you get to 2mb you suddenly become aware of the speed limit of the web...having a 2mb connection means you can download at about 256 kilobytes a second maximum but in reality, you will be lucky to ever go above 150k.