/20 Subnet Mask ReferenceIPv4
Sixteen /24s under <code>255.255.240.0</code>: a /20 delivers 4,094 usable hosts, the sweet spot for a large office floor, a busy warehouse, or a cloud subnet that refuses to waste space.
/20 facts at a glance
- Subnet mask
- 255.255.240.0
- Wildcard mask
- 0.0.15.255
- Binary mask
- 11111111.11111111.11110000.00000000
- Total addresses
- 4,096
- Usable hosts
- 4,094 (excludes network & broadcast)
- Block size
- 4,096 addresses (212)
- Typical role
- Mid-size networks and ISP allocations
Example /20 networks
Three consecutive /20 blocks, starting at the /20 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.51.96.0/20 | 198.51.96.1 – 198.51.111.254 | 4,094 |
| 198.51.112.0/20 | 198.51.112.1 – 198.51.127.254 | 4,094 |
| 198.51.128.0/20 | 198.51.128.1 – 198.51.143.254 | 4,094 |
How to calculate a /20 mask
A /20 mask sets the first 20 bits of a 32-bit number to 1 and leaves the remaining 12 bits as 0. In binary that reads 11111111.11111111.11110000.00000000, which converts to dotted decimal 255.255.240.0. Each block covers a contiguous run of 212 = 4,096 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.0.15.255.
Where /20 is used
- One large-office production segment carrying thousands of endpoints.
- Cloud subnets sized with headroom for autoscaling fleets.
- Generous DHCP scopes where lease churn is high and exhaustion is unacceptable.
A /20 tiles sixteen times into a /16 and contains exactly sixteen /24s; its third octet advances in steps of 16.
Frequently asked questions
How many usable hosts are in a /20?
4,094 of 4,096 total addresses; the network and broadcast slots account for the difference.
What is the subnet mask for a /20?
<code>255.255.240.0</code>, wildcard <code>0.0.15.255</code>.
How many /24s are in a /20?
Sixteen consecutive ones, which is why a /20 is often described informally as sixteen class Cs.
Is a /20 a good office size?
For most large offices, yes: roughly four thousand devices with room to grow. Watch broadcast traffic and DHCP capacity if you flatten it into a single segment rather than VLAN-per-floor.
Open this block in a calculator
Open the /20 block in the subnet calculator