What is a /25 subnet? 128 IPs, and two fit in a /24

How this is calculated

The mask 255.255.255.128 leaves 7 host bits, so each /25 holds 128 addresses. A /24 holds 2 of these blocks, so the two halves can serve different segments without a second address plan. The first runs from 192.168.1.0 to 192.168.1.127, and its usable range is 192.168.1.1 to 192.168.1.126. The second block starts at 192.168.1.128, and its usable range is 192.168.1.129 to 192.168.1.254. AWS and Azure both allow a /25 and both reserve five addresses per subnet, so a cloud subnet of this size keeps 123 addresses for resources. One /24 can serve staff and guests if each gets a /25 and a firewall rule separates the two.

Verdict

Pick a /25 for a guest network, a small office VLAN or a lab that needs dozens of devices and has room to grow. The 126 usable addresses cover that, but check the headroom before you commit, since device counts can climb over time. Keep the DHCP pool to part of the block, so static devices like printers and access points have addresses that DHCP won't hand out. Count the addresses already taken by the gateway and any static servers before you settle on the mask. If the guest network outgrows its /25, it needs a bigger block, and that only works if the other half of the /24 is still free. If you want to split the block later, a /25 splits into 2 /26 blocks.