/12 Subnet Mask ReferenceIPv4
Sixteen /16 networks bundled under <code>255.240.0.0</code>: a /12 offers 1,048,574 usable hosts and anchors one of the most recognized lines in networking, the private range <code>172.16.0.0/12</code> from RFC 1918.
/12 facts at a glance
- Subnet mask
- 255.240.0.0
- Wildcard mask
- 0.15.255.255
- Binary mask
- 11111111.11110000.00000000.00000000
- Total addresses
- 1,048,576
- Usable hosts
- 1,048,574 (excludes network & broadcast)
- Block size
- 1,048,576 addresses (220)
- Typical role
- Large enterprise and carrier aggregation
Example /12 networks
Three consecutive /12 blocks, starting at the /12 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.48.0.0/12 | 198.48.0.1 – 198.63.255.254 | 1,048,574 |
| 198.64.0.0/12 | 198.64.0.1 – 198.79.255.254 | 1,048,574 |
| 198.80.0.0/12 | 198.80.0.1 – 198.95.255.254 | 1,048,574 |
How to calculate a /12 mask
A /12 mask sets the first 12 bits of a 32-bit number to 1 and leaves the remaining 20 bits as 0. In binary that reads 11111111.11110000.00000000.00000000, which converts to dotted decimal 255.240.0.0. Each block covers a contiguous run of 220 = 1,048,576 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.15.255.255.
Where /12 is used
- Private addressing at scale: 172.16.0.0 through 172.31.255.255 is exactly one /12, a favorite for enterprises that want a second private pool alongside 10.x.x.x.
- Region-sized internal zones for organizations juggling hundreds of /16s.
- Container ecosystems: popular runtimes default into slices of this range; Docker's standard bridge network uses 172.17.0.0/16.
Neighbor, not subset: carrier-grade NAT lives in <code>100.64.0.0/10</code> (RFC 6598). That /10 is four times the size of a /12 and counts as shared address space, not RFC 1918 private space.
A /12 divides into 16 /16s or 4,096 /24s, which makes hierarchical planning inside it straightforward.
Frequently asked questions
What is 172.16.0.0/12?
The middle RFC 1918 private range, covering 172.16.0.0 through 172.31.255.255. Devices may reuse it freely internally, but it is not routable on the public Internet.
How many hosts are in a /12?
1,048,574 usable of 1,048,576 total; the first and last addresses serve as network and broadcast.
What mask goes with a /12?
<code>255.240.0.0</code>, wildcard <code>0.15.255.255</code>. The twelve fixed bits reach four bits into the second octet.
Why do my containers get 172.17.x.x addresses?
Runtime defaults fall inside the 172.16.0.0/12 private block. If that collides with your corporate network, change the runtime's address pool rather than the corporate plan.
Open this block in a calculator
Open the /12 block in the subnet calculator