/25 Subnet Mask

A /25 subnet mask is 255.255.255.128. It has 128 addresses, 126 usable hosts and the wildcard mask 0.0.0.127.

CIDR prefix/25
Subnet mask255.255.255.128
Wildcard mask0.0.0.127
Binary mask11111111.11111111.11111111.10000000
Total addresses128
Usable hosts126
Block size128 (fourth octet)

Example: 192.168.1.0/25

Network address192.168.1.0
First usable host192.168.1.1
Last usable host192.168.1.126
Broadcast address192.168.1.127

Where /25 subnets start

/25 networks start on multiples of 128 in the fourth octet (0, 128, …). A /24 holds 2 /25 subnets, and a /16 holds 512.

How to calculate a /25 subnet

  1. Host bits: 32 − 25 = 7.
  2. Total addresses: 27 = 128.
  3. Usable hosts: 128 − 2 = 126, because the network and broadcast addresses can't be assigned to hosts.
  4. Subnet mask: 25 one-bits followed by 7 zero-bits. The fourth octet is 128 (256 − 128), which gives 255.255.255.128.
  5. Wildcard mask: 255.255.255.255 − 255.255.255.128 = 0.0.0.127.

When to use a /25

A /25 splits a /24 into two subnets of 126 usable hosts each.

Updated . Calculate any network with the subnet calculator, or compare all prefixes in the subnet cheat sheet.