/32 Subnet Mask ReferenceIPv4
One address, no neighbors: a /32 is a route to a single machine. Firewalls love its precision, BGP sessions ride it toward loopback interfaces, and cloud consoles quietly store every single-IP allowlist entry you type as one.
/32 facts at a glance
- Subnet mask
- 255.255.255.255
- Wildcard mask
- 0.0.0.0
- Binary mask
- 11111111.11111111.11111111.11111111
- Total addresses
- 1
- Usable hosts
- 1 (host route)
- Block size
- 1 addresses (20)
- Typical role
- Single-host route
Example /32 networks
Three consecutive /32 blocks, starting at the /32 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.51.100.0/32 | 198.51.100.0 | 1 |
| 198.51.100.1/32 | 198.51.100.1 | 1 |
| 198.51.100.2/32 | 198.51.100.2 | 1 |
How to calculate a /32 mask
A /32 mask sets the first 32 bits of a 32-bit number to 1 and leaves the remaining 0 bits as 0. In binary that reads 11111111.11111111.11111111.11111111, which converts to dotted decimal 255.255.255.255. Each block covers a contiguous run of 20 = 1 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.0.0.0.
Where /32 is used
- Loopback interfaces on routers: stable, interface-independent endpoints for management SSH and BGP peering.
- Firewall and ACL rules permitting exactly one address, using mask <code>255.255.255.255</code> and wildcard <code>0.0.0.0</code>.
- Host routes pinned into routing tables for blackholing or traffic steering.
- AWS security groups: allowing a lone IP stores the rule as that address followed by /32.
A /32 has no network or broadcast twin. The single address is simultaneously the network, the broadcast, and the host, so the usable count is simply 1.
Peering to partners' /32 loopbacks keeps BGP sessions alive even when individual physical interfaces fail, since the route follows whatever path reaches that loopback.
Frequently asked questions
What is a /32 IP address?
A single, fully specified address. The mask <code>255.255.255.255</code> fixes all thirty-two bits, so the prefix identifies one exact host and nothing else.
How many usable hosts are in a /32?
Exactly one. There are no reserved companion addresses to subtract.
Why do routers use /32 loopback interfaces?
A loopback never goes down when a cable does, so management sessions and BGP peerings anchored to its /32 survive physical failures.
What does adding /32 after an IP mean in a firewall rule?
It states that the rule matches that single address only, rather than a range. Omitting it invites ambiguity; including it makes intent explicit.
Open this block in a calculator
Open the /32 block in the subnet calculator