The /20 subnet
A /20 subnet uses the mask 255.255.240.0 and holds 4,096 addresses, 4,094 of them usable.
A /20 sits between a /16 and a /24: 4,096 addresses, 4,094 usable, mask 255.255.240.0. It is a common size for a large flat segment or for aggregating several /24s into one route.
Summarizing contiguous /24s into a /20 keeps routing tables small. The block still fits inside a single broadcast domain if you want it to, though most designs subnet it further.
Below you can see the exact network, broadcast, and host range for any /20 you enter, calculated instantly in your browser.
Calculate a /20 network
Accepts CIDR (192.0.2.0/24), IP + mask (192.0.2.0 255.255.255.0), or a bare IP (defaults to /24).
11000000.00000000.00000010.0000000011111111.11111111.11110000.00000000Example: 192.0.2.0/20
Taking 192.0.2.0/20 as a worked example, the block breaks down like this:
| Network address | 192.0.0.0 |
|---|---|
| Usable range | 192.0.0.1 - 192.0.15.254 |
| Broadcast address | 192.0.15.255 |
| Total addresses | 4,096 |
| Usable hosts | 4,094 |
| Size | 16 class C blocks |
Frequently asked questions
What is the subnet mask for a /20?
A /20 uses the subnet mask 255.255.240.0. Its wildcard mask, used in access lists, is 0.0.15.255.
How many hosts are in a /20 subnet?
A /20 has 4,096 total addresses and 4,094 usable hosts (the total minus the network and broadcast addresses, except for /31 and /32).
What is a /20 used for?
Standard LAN size. It is equivalent to 16 class C blocks.
How do I calculate a /20 network?
Set the host bits to zero for the network address and to one for the broadcast address. The calculator on this page does it for any /20 block you enter.