The /25 prefix means the first 25 bits of the address identify the network and the remaining 7 identify the host. Written out, the mask is 255.255.255.128, in binary 11111111.11111111.11111111.10000000, and its Cisco wildcard or inverse form is 0.0.0.127. It covers 128 addresses, of which 126 can be given to hosts.
| CIDR prefix | /25 |
|---|---|
| Subnet mask | 255.255.255.128 |
| Mask in binary | 11111111.11111111.11111111.10000000 |
| Wildcard (inverse) mask | 0.0.0.127 |
| Host bits | 7 |
| Total addresses | 128 |
| Usable hosts | 126 |
| Block size (magic number) | 128 in octet 4 |
A 192.168.1.0/24 splits into exactly 2 subnets of this size.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 192.168.1.0/25 | 192.168.1.1 | 192.168.1.126 | 192.168.1.127 |
| 192.168.1.128/25 | 192.168.1.129 | 192.168.1.254 | 192.168.1.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /25 that is 128 in octet 4. Subtract it from 256:
256 - 128 = 128
That is the block size. Every /25 network address is a multiple of 128 in octet 4, so you can list them by counting: 0, 128, 256, 384, and so on. No binary needed once you know the number.
A /25 is a /24 cut in half, 126 usable addresses each. The usual reason is separating two classes of device that were sharing a VLAN, voice from data being the classic pair, without asking for a new range.
A /25 holds 128 addresses in total. After reserving the network and broadcast addresses, 126 are usable by hosts.
The /25 subnet mask is 255.255.255.128 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.0.0.127. It is the bitwise inverse of 255.255.255.128 and is what Cisco access lists and OSPF network statements expect.
The /25 mask written out bit by bit is 11111111.11111111.11111111.10000000: 25 ones followed by 7 zeros.
A /24 splits into 2 subnets of /25.