/12 Subnet Mask
A /12 prefix means the subnet mask is 255.240.0.0. It has 20 host bits, giving 1,048,576 total addresses and 1,048,574 usable hosts.
/12 at a glance
| Property | Value |
|---|---|
| CIDR prefix | /12 |
| Subnet mask | 255.240.0.0 |
| Wildcard mask | 0.15.255.255 |
| Total addresses | 1,048,576 |
| Usable hosts | 1,048,574 |
| Host bits | 20 |
How it is calculated
A /12 sets the first 12 bits to 1 (network) and the remaining 20 bits to 0 (host). Usable hosts = 220 − 2 = 1,048,574.
The wildcard mask 0.15.255.255 is the inverse — used in Cisco ACLs and OSPF network statements.
Frequently asked questions
What is a /12 subnet mask?
A /12 equals 255.240.0.0 in dotted decimal (wildcard 0.15.255.255). It provides 1,048,576 total addresses and 1,048,574 usable hosts.
How many hosts are in a /12?
A /12 has 20 host bits, so it supports 1,048,574 usable hosts.
What is the wildcard mask for /12?
The wildcard (inverse) mask for /12 is 0.15.255.255, used in Cisco ACLs and OSPF statements.
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