ip address range

an ip address is divided into network id and host id..! everyone knows that. for a class A ip address the range is 2 power 7 i.e., 127… since the 8th bit of first octet is set to 0 and hence remaining are 7 bits so 2 is powered by 7 and range is 127 max.
how is the range calculated for class B and class C?? rnage of class B is from 128.0.0.0 to 191.255.25.255.. how is it determined tat range of class B is from 127 to 191?? how is it calculated..? can any one plzzz help me??

thanks in advance..!!