Why it is said that we cannot connect more than 127 usb devices to our computer(in usb 2.0) ?

Each device has its own ID. USB packets have are structured in a way that only allows 7 bits for an address. That allows for 128 combinations, or 0-127. I think I heard that the 0 ID is reserved. This would mean that you have 127 unique ID left to be given to the devices.