/1 Subnet: Mask 128.0.0.0, 2,147,483,646 Usable Hosts

A /1 subnet has a subnet mask of 128.0.0.0 and contains 2,147,483,648 total IP addresses, of which 2,147,483,646 are usable host addresses (the network and broadcast addresses are reserved). A /1 contains 8,388,608 /24 networks.

Usable hosts in a /1 by platform

Cloud providers reserve more than the usual two addresses per subnet, so the same /1 gives you fewer usable IPs in the cloud than on a standard LAN.

Good to know

A /1 splits the whole internet into two halves. It is almost never used for real allocations, but it appears in routing tricks and traffic-engineering demos.

How a /1 is calculated

• The /1 means the first 1 bits are the network portion, leaving 31 host bits.
• Total addresses = 2^(32 - 1) = 2^31 = 2,147,483,648.
• Usable hosts (standard) = 2,147,483,648 - 2 (network + broadcast) = 2,147,483,646.
• Subnet mask = 128.0.0.0; wildcard mask = 127.255.255.255.