/1 Subnet Mask
A /1 prefix means the subnet mask is 128.0.0.0. It has 31 host bits, giving 2,147,483,648 total addresses and 2,147,483,646 usable hosts.
/1 at a glance
| Property | Value |
|---|---|
| CIDR prefix | /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 |
How it is calculated
A /1 sets the first 1 bits to 1 (network) and the remaining 31 bits to 0 (host). Usable hosts = 231 − 2 = 2,147,483,646.
The wildcard mask 127.255.255.255 is the inverse — used in Cisco ACLs and OSPF network statements.
Frequently asked questions
What is a /1 subnet mask?
A /1 equals 128.0.0.0 in dotted decimal (wildcard 127.255.255.255). It provides 2,147,483,648 total addresses and 2,147,483,646 usable hosts.
How many hosts are in a /1?
A /1 has 31 host bits, so it supports 2,147,483,646 usable hosts.
What is the wildcard mask for /1?
The wildcard (inverse) mask for /1 is 127.255.255.255, used in Cisco ACLs and OSPF statements.
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