/1 subnet (128.0.0.0)
A /1 network has 2,147,483,648 addresses, of which 2,147,483,646 can be given to hosts after the network and broadcast addresses. A huge block covering a large share of the IPv4 address space, seen in routing and broad firewall rules.
- CIDR notation
- /1
- Subnet mask
- 128.0.0.0
- Wildcard mask
- 127.255.255.255
- Binary mask
- 10000000.00000000.00000000.00000000
- Total addresses
- 2,147,483,648
- Usable hosts
- 2,147,483,646
- Example network
- 0.0.0.0/1
- Example usable range
- 0.0.0.1 – 127.255.255.254
- /24 networks in a /1
- 8,388,608
Calculate a /1 for your own address
In firewall rules and investigations
A /1 rule matches every address from 0.0.0.0 to 127.255.255.255. In Cisco ACLs the same range is written with the wildcard mask 127.255.255.255. When an alert names an IP, checking whether it falls inside a known /1 (your own network, a cloud provider’s published range or a blocked range) is often the quickest first step.