Class B
Class B IP addresses are assigned to large and medium size companies.
The IANA specifies that Class B addresses will lead with 10 when written in binary format. This means that the range for the first octet of Class B addresses is 128 through 191.
With the first two binary digits of the first octet in the Class B category defined, the address range is limited.
When the last six configurable bits of the first octet are set to zero, the lowest configurable number is obtained, which is 128.
When those same six digits are set to one, the highest configurable number is set, which is 191.