Assuming that you know your bitwise numbering system, the base of 2.
255.255.255.128 is equal to this in base 2
Looking at the mask in bitwise form, the 7 zeros on the right will represent the host address, the rest of the digits to the left represent the network address. So starting from 1, the computers can be assigned in bitwise, 00001, 00010, 00011, 00100, 00101, and so on until you finsihed playing with all 7 digits. The computer's address can't be all 0's or all 1's. So, first address will be 0000001, and last address would be 1111110. In base 10, that would be from 1 to 126, giving us 126 addresses assignable to host.