Subnet

Host (ISP): 151.151.0.0/25
First subnet: 151.151.0.128 ! why not 151.151.0.0? Because it assigned to Host(ISP) it is not a subnet.
2nd: 151.151.1.0
...

Host range: 1st: 151.151.0.129 - 151.151.0.254  (255 is Broadcast)
151.151.0.1-151.151.0.126 are belong to 151.151.0.0.