The /1 subnet
A /1 subnet uses the mask 128.0.0.0 and holds 2,147,483,648 addresses, 2,147,483,646 of them usable.
A /1 is a very large IPv4 block. Its mask is 128.0.0.0 and it holds 2,147,483,648 addresses, which puts it at the scale of a regional allocation or an entire private supernet rather than a single LAN.
Networks this size are almost always carved into smaller subnets before use. You will see a /1 as an aggregate route or a private-address supernet such as 10.0.0.0/8, not as a single broadcast domain.
Below you can see the exact network, broadcast, and host range for any /1 you enter, calculated instantly in your browser.
Calculate a /1 network
Accepts CIDR (192.0.2.0/24), IP + mask (192.0.2.0 255.255.255.0), or a bare IP (defaults to /24).
00001010.00000000.00000000.0000000010000000.00000000.00000000.00000000Example: 10.0.0.0/1
Taking 10.0.0.0/1 as a worked example, the block breaks down like this:
| Network address | 0.0.0.0 |
|---|---|
| Usable range | 0.0.0.1 - 127.255.255.254 |
| Broadcast address | 127.255.255.255 |
| Total addresses | 2,147,483,648 |
| Usable hosts | 2,147,483,646 |
| Size | 128 class A blocks |
Frequently asked questions
What is the subnet mask for a /1?
A /1 uses the subnet mask 128.0.0.0. Its wildcard mask, used in access lists, is 127.255.255.255.
How many hosts are in a /1 subnet?
A /1 has 2,147,483,648 total addresses and 2,147,483,646 usable hosts (the total minus the network and broadcast addresses, except for /31 and /32).
What is a /1 used for?
Very large network. It is equivalent to 128 class A blocks.
How do I calculate a /1 network?
Set the host bits to zero for the network address and to one for the broadcast address. The calculator on this page does it for any /1 block you enter.