/25 Subnet: Mask 255.255.255.128, 126 Usable Hosts

A /25 subnet has a subnet mask of 255.255.255.128 and contains 128 total IP addresses, of which 126 are usable host addresses (the network and broadcast addresses are reserved). 2 /25 networks fit inside a single /24.

Usable hosts in a /25 by platform

Cloud providers reserve more than the usual two addresses per subnet, so the same /25 gives you fewer usable IPs in the cloud than on a standard LAN.

Good to know

A /25 (126 usable hosts) is the result of splitting a /24 in half. Common when you want two separate subnets out of one Class C.

How a /25 is calculated

• The /25 means the first 25 bits are the network portion, leaving 7 host bits.
• Total addresses = 2^(32 - 25) = 2^7 = 128.
• Usable hosts (standard) = 128 - 2 (network + broadcast) = 126.
• Subnet mask = 255.255.255.128; wildcard mask = 0.0.0.127.