# /21 Subnet Mask (255.255.248.0) – Hosts, Range & FAQ

> The /21 subnet mask is 255.255.248.0. A /21 block holds 2,048 addresses with 2,046 usable hosts — ranges, wildcard, binary math and FAQs.

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# /21 Subnet Mask ReferenceIPv4

Eight neighboring /24s summarized as one: mask 255.255.248.0, 2,048 addresses. Operators like the /21 as a unit for handing out public space in politely aggregatable chunks.

The /21 subnet mask is 255.255.248.0, equivalent to 0.0.7.255 as a wildcard. A /21 block contains 2,048 IP addresses (2,046 usable hosts).

## /21 facts at a glance

- **Subnet mask:** 255.255.248.0
- **Wildcard mask:** 0.0.7.255
- **Binary mask:** 11111111.11111111.11111000.00000000
- **Total addresses:** 2,048
- **Usable hosts:** 2,046 (excludes network & broadcast)
- **Block size:** 2,048 addresses (211)
- **Typical role:** Mid-size networks and ISP allocations

## Example /21 networks

Three consecutive /21 blocks, starting at the /21 boundary that contains 198.51.100.0:

| Network | Usable range | Usable hosts |
| --- | --- | --- |
| 198.51.96.0/21 | 198.51.96.1 – 198.51.103.254 | 2,046 |
| 198.51.104.0/21 | 198.51.104.1 – 198.51.111.254 | 2,046 |
| 198.51.112.0/21 | 198.51.112.1 – 198.51.119.254 | 2,046 |

## How to calculate a /21 mask

A /21 mask sets the first 21 bits of a 32-bit number to 1 and leaves the remaining 11 bits as 0. In binary that reads `11111111.11111111.11111000.00000000`, which converts to dotted decimal `255.255.248.0`. Each block covers a contiguous run of 211 = 2,048 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving `0.0.7.255`.

## Where /21 is used

- ISP customer blocks that fold back into one upstream announcement.
- Mid-size sites needing several VLANs under a single allocation.
- Compact public ranges for hosting clusters.

Documentation landmark: TEST-NET-3, 203.0.113.0/24 from RFC 5737, sits inside the aligned block 203.0.112.0/21.

## Frequently asked questions

**What is the subnet mask for /21?**

255.255.248.0, wildcard 0.0.7.255.

**How many addresses are in a /21?**

2,048 total and 2,046 usable after the standard two-address deduction.

**How many /24s are in a /21?**

Eight; the third octet steps through multiples of eight within the block.

## Open this block in a calculator

[Open the /21 block in the subnet calculator](https://itinsighthub.com/subnet-calculator/?q=198.51.96.0%2F21)

[Get the /21 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F21)

### Related prefixes

[/20](https://itinsighthub.com/subnet/20/)[/22](https://itinsighthub.com/subnet/22/)[/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/30](https://itinsighthub.com/subnet/30/)

← /20 subnet mask
/22 subnet mask →

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