Subnet Mask
/20 Subnet Mask
A /20 prefix means the subnet mask is 255.255.240.0. It has 12 host bits, giving 4,096 total addresses and 4,094 usable hosts.
/20 at a glance
| Property | Value |
|---|---|
| CIDR prefix | /20 |
| Subnet mask | 255.255.240.0 |
| Wildcard mask | 0.0.15.255 |
| Total addresses | 4,096 |
| Usable hosts | 4,094 |
| Host bits | 12 |
How it is calculated
A /20 sets the first 20 bits to 1 (network) and the remaining 12 bits to 0 (host). Usable hosts = 212 − 2 = 4,094.
The wildcard mask 0.0.15.255 is the inverse — used in Cisco ACLs and OSPF network statements.
Frequently asked questions
What is a /20 subnet mask?
A /20 equals 255.255.240.0 in dotted decimal (wildcard 0.0.15.255). It provides 4,096 total addresses and 4,094 usable hosts.
How many hosts are in a /20?
A /20 has 12 host bits, so it supports 4,094 usable hosts.
What is the wildcard mask for /20?
The wildcard (inverse) mask for /20 is 0.0.15.255, used in Cisco ACLs and OSPF statements.
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