How can I store numbers > 128 in a signed one byte interger?

I am reading the book Art of Assembly Language. There I came across this paragraph.

If the H.O. bit is zero, then the number is positive and is stored as a standard binary value. If the H.O. bit is one, then the number is negative and is stored in the two’s comple-ment form. To convert a positive number to its negative, two’s complement form, you use the following algorithm:

1. Invert all the bits in the number, i.e., apply the logical NOT function.

2. Add one to the inverted result.

   For example, to compute the eight bit equivalent of -5:

   0000 0101  Five (in binary)   
   1111 1010  Invert all the bits.   
   1111 1011  Add one to obtain result.
   

Here I want to know if 0000 0101 is 5 in decimals and 1111 1011 is -5 then how we represent 251? Is not the same 1111 1011? How the computer distinguishes between -5 and 251?

When you are representing signed numbers in 8 bits, the 8th bit (the HO bit) is the sign bit. Therefore you can only use 7 bits to store the value of the number. The range for a signed number in 8 bits is -128..127. 251 can't be represented, unless you use more than 8 bits.