Subnet Mask
/25 Subnet Mask
A /25 prefix means the subnet mask is 255.255.255.128. It has 7 host bits, giving 128 total addresses and 126 usable hosts.
/25 at a glance
| Property | Value |
|---|---|
| CIDR prefix | /25 |
| Subnet mask | 255.255.255.128 |
| Wildcard mask | 0.0.0.127 |
| Total addresses | 128 |
| Usable hosts | 126 |
| Host bits | 7 |
How it is calculated
A /25 sets the first 25 bits to 1 (network) and the remaining 7 bits to 0 (host). Usable hosts = 27 − 2 = 126.
The wildcard mask 0.0.0.127 is the inverse — used in Cisco ACLs and OSPF network statements.
Frequently asked questions
What is a /25 subnet mask?
A /25 equals 255.255.255.128 in dotted decimal (wildcard 0.0.0.127). It provides 128 total addresses and 126 usable hosts.
How many hosts are in a /25?
A /25 has 7 host bits, so it supports 126 usable hosts.
What is the wildcard mask for /25?
The wildcard (inverse) mask for /25 is 0.0.0.127, used in Cisco ACLs and OSPF statements.
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