/1 subnet calculator (128.0.0.0)
A /1 prefix fixes the first 1 bits of the address, leaving 31 bits for hosts. That gives 2,147,483,648 addresses in the block.
Usable hosts per subnet
2 147 483 646
- Total addresses in the block
- 2 147 483 648
- Mask octet 1
- 128
- Mask octet 2
- 0
- Mask octet 3
- 0
- Mask octet 4
- 0
- Usable hosts across all the blocks
- 2 147 483 646
- Blocks needed for the hosts you listed
- 1
- Spare addresses in one block
- 2 147 483 596
- Utilisation of one block
- 0.00 %
- How many fit inside a /24
- 0
Show the working
- Number of these subnets = 1
- Hosts you need to fit = 50
- prefix = 1 = 1
- host_bits = 32 - 1 = 31
- Total addresses in the block = 2^31 = 2 147 480 000
- Usable hosts per subnet = 1 >= 31 ? (1 == 31 ? 2 : 1) : 2 147 480 000 - 2 = 2 147 480 000
- mask_value = 2^32 - 2^31 = 2 147 480 000
- Mask octet 1 = floor(2 147 480 000 / 2^24) = 128
- Mask octet 2 = floor(2 147 480 000 / 2^16) % 256 = 0
- Mask octet 3 = floor(2 147 480 000 / 2^8) % 256 = 0
- Mask octet 4 = 2 147 480 000 % 256 = 0
- total_addresses = 2 147 480 000 * 1 = 2 147 480 000
- Usable hosts across all the blocks = 2 147 480 000 * 1 = 2 147 480 000
- Blocks needed for the hosts you listed = 2 147 480 000 > 0 ? ceil(50 / 2 147 480 000) : 0 = 1
- Spare addresses in one block = 2 147 480 000 - 50 = 2 147 480 000
- Utilisation of one block = 2 147 480 000 > 0 ? 50 / 2 147 480 000 * 100 : 0 = 0.000 002 328 31
- How many fit inside a /24 = 1 >= 24 ? 2^(1 - 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 | /1 |
| Subnet mask | 128.0.0.0 |
| Wildcard mask | 127.255.255.255 |
| Total addresses | 2,147,483,648 |
| Usable hosts | 2,147,483,646 |
| Host bits | 31 |
Frequently asked
How many hosts are in a /1?
2,147,483,646 usable hosts, out of 2,147,483,648 total addresses.
What subnet mask is a /1?
128.0.0.0, with a wildcard mask of 127.255.255.255.
Where the numbers come from
- Address arithmetic follows RFC 4632 (CIDR) and RFC 3021 for the /31 case.