/27 subnet calculator (255.255.255.224)
A /27 prefix fixes the first 27 bits of the address, leaving 5 bits for hosts. That gives 32 addresses in the block.
Usable hosts per subnet
30
- Total addresses in the block
- 32
- Mask octet 1
- 255
- Mask octet 2
- 255
- Mask octet 3
- 255
- Mask octet 4
- 224
- Usable hosts across all the blocks
- 30
- Blocks needed for the hosts you listed
- 2
- Spare addresses in one block
- -20
- Utilisation of one block
- 166.67 %
- How many fit inside a /24
- 8
Show the working
- Number of these subnets = 1
- Hosts you need to fit = 50
- prefix = 27 = 27
- host_bits = 32 - 27 = 5
- Total addresses in the block = 2^5 = 32
- Usable hosts per subnet = 27 >= 31 ? (27 == 31 ? 2 : 1) : 32 - 2 = 30
- mask_value = 2^32 - 2^5 = 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 = 255
- Mask octet 4 = 4 294 970 000 % 256 = 224
- total_addresses = 32 * 1 = 32
- Usable hosts across all the blocks = 30 * 1 = 30
- Blocks needed for the hosts you listed = 30 > 0 ? ceil(50 / 30) : 0 = 2
- Spare addresses in one block = 30 - 50 = -20
- Utilisation of one block = 30 > 0 ? 50 / 30 * 100 : 0 = 166.667
- How many fit inside a /24 = 27 >= 24 ? 2^(27 - 24) : 0 = 8
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 | /27 |
| Subnet mask | 255.255.255.224 |
| Wildcard mask | 0.0.0.31 |
| Total addresses | 32 |
| Usable hosts | 30 |
| Host bits | 5 |
Frequently asked
How many hosts are in a /27?
30 usable hosts, out of 32 total addresses.
What subnet mask is a /27?
255.255.255.224, with a wildcard mask of 0.0.0.31.
Where the numbers come from
- Address arithmetic follows RFC 4632 (CIDR) and RFC 3021 for the /31 case.