/21 subnet calculator (255.255.248.0)
A /21 prefix fixes the first 21 bits of the address, leaving 11 bits for hosts. That gives 2,048 addresses in the block.
Usable hosts per subnet
2046
- Total addresses in the block
- 2048
- Mask octet 1
- 255
- Mask octet 2
- 255
- Mask octet 3
- 248
- Mask octet 4
- 0
- Usable hosts across all the blocks
- 2046
- Blocks needed for the hosts you listed
- 1
- Spare addresses in one block
- 1996
- Utilisation of one block
- 2.44 %
- How many fit inside a /24
- 0
Show the working
- Number of these subnets = 1
- Hosts you need to fit = 50
- prefix = 21 = 21
- host_bits = 32 - 21 = 11
- Total addresses in the block = 2^11 = 2048
- Usable hosts per subnet = 21 >= 31 ? (21 == 31 ? 2 : 1) : 2048 - 2 = 2046
- mask_value = 2^32 - 2^11 = 4 294 970 000
- Mask octet 1 = floor(4 294 970 000 / 2^24) = 255
- Mask octet 2 = floor(4 294 970 000 / 2^16) % 256 = 255
- Mask octet 3 = floor(4 294 970 000 / 2^8) % 256 = 248
- Mask octet 4 = 4 294 970 000 % 256 = 0
- total_addresses = 2048 * 1 = 2048
- Usable hosts across all the blocks = 2046 * 1 = 2046
- Blocks needed for the hosts you listed = 2046 > 0 ? ceil(50 / 2046) : 0 = 1
- Spare addresses in one block = 2046 - 50 = 1996
- Utilisation of one block = 2046 > 0 ? 50 / 2046 * 100 : 0 = 2.443 79
- How many fit inside a /24 = 21 >= 24 ? 2^(21 - 24) : 0 = 0
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- host bits = 32 - prefix length
- total addresses = 2^host bits
- usable hosts = total - 2, reserving the network and broadcast addresses
- mask = 2^32 - 2^host bits, written as four octets
The two reserved addresses are the all-zeros network address and the all-ones broadcast address.
RFC 3021 makes a /31 usable as a two-address point-to-point link with no broadcast, and a /32 is a single host route. Both are special-cased here.
This is IPv4. IPv6 does not reserve a broadcast address, so an IPv6 prefix has 2^(128 - n) usable addresses.
This prefix at a glance
| Property | Value |
|---|---|
| CIDR | /21 |
| Subnet mask | 255.255.248.0 |
| Wildcard mask | 0.0.7.255 |
| Total addresses | 2,048 |
| Usable hosts | 2,046 |
| Host bits | 11 |
Frequently asked
How many hosts are in a /21?
2,046 usable hosts, out of 2,048 total addresses.
What subnet mask is a /21?
255.255.248.0, with a wildcard mask of 0.0.7.255.
Where the numbers come from
- Address arithmetic follows RFC 4632 (CIDR) and RFC 3021 for the /31 case.