# /12 Subnet Mask (255.240.0.0) – Hosts, Range & FAQ

> The /12 subnet mask is 255.240.0.0. A /12 block holds 1,048,576 addresses with 1,048,574 usable hosts — ranges, wildcard, binary math and FAQs.

- Page URL: https://itinsighthub.com/subnet/12/
- Markdown variant of this page: append `?format=md` to any URL on this site or send `Accept: text/markdown`.
- Full site index for AI assistants: https://itinsighthub.com/llms.txt
- Full site export: https://itinsighthub.com/llms-full.txt

# /12 Subnet Mask ReferenceIPv4

Sixteen /16 networks bundled under 255.240.0.0: a /12 offers 1,048,574 usable hosts and anchors one of the most recognized lines in networking, the private range 172.16.0.0/12 from RFC 1918.

The /12 subnet mask is 255.240.0.0, equivalent to 0.15.255.255 as a wildcard. A /12 block contains 1,048,576 IP addresses (1,048,574 usable hosts).

## /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 100.64.0.0/10 (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?**

255.240.0.0, wildcard 0.15.255.255. 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](https://itinsighthub.com/subnet-calculator/?q=198.48.0.0%2F12)

[Get the /12 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F12)

### Related prefixes

[/11](https://itinsighthub.com/subnet/11/)[/13](https://itinsighthub.com/subnet/13/)[/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/30](https://itinsighthub.com/subnet/30/)

← /11 subnet mask
/13 subnet mask →

---

© 2026 ITInsightHub · [About](https://itinsighthub.com/about/) · [Contact](https://itinsighthub.com/contact/) · [Privacy](https://itinsighthub.com/privacy/)