What could be the cause when the first half of the C-class IP work, the other half not work?

Question

For example, the IP xxx.xxx.xxx.1-127 works, but xxx.xxx.xxx.128-251 are not working. They are working before. Thanks.

Answer 1

The first thing that comes to mind is an incorrect subnet mask somewhere, but you really need to give more information. What exactly are you doing?