The 255.255.255.128 subnet mask.

The /25 subnet uses 255.255.255.128 as its subnet mask — meaning the first 25 bits of every address identify the network, and the remaining 7 bits identify the host within that network. That gives you 128 total addresses (126 usable on standard RFC math, after subtracting the network and broadcast addresses).

The wildcard mask — the bitwise inverse of the subnet mask — is 0.0.0.127. Wildcards are what Cisco access-control lists and OSPF area definitions use instead of subnet masks; the "1" bits mark "don't care" positions. For a /25, that leaves 7 don't-care host bits.

To find the network address for any IP in a /25 block, perform a bitwise AND between the IP and the subnet mask. To find the broadcast, OR the network address with the wildcard. Modern tools — like our subnet calculator — do this in microseconds, but the underlying mechanics are straightforward binary arithmetic.

A /25 splits a /24 in half. 126 usable hosts (123 on AWS / Azure, 124 on GCP). Useful for separating server and client tiers, or for keeping two VLANs in one /24's worth of address space.

Cloud-provider quirks matter at every prefix size: AWS and Azure reserve 5 IPs per subnet, GCP reserves 4, and OCI reserves 3. So a /25 on standard RFC math gives you 126 usable hosts, but on AWS or Azure that drops to 123. The capacity-planning gap bites hardest at small prefixes (a /28 has 14 usable on paper, only 11 on AWS) but exists at every size. Our cloud-aware calculator applies the right math automatically.