Why does underflow with floating points happen at 2^-126?

When representing a float, why does the exponent face underflow when it hits 2^-126 if 8 bits can hold everything from -127(incl.) to 128(incl.)?

>Solution :

Exponents range from −126 to +127 because exponents of −127 (all 0s) and +128 (all 1s) are reserved for special numbers. wikipedia