What is a /32 Subnet?

Last reviewed: May 2026

A /32 is fundamentally different from all other prefix lengths — it is not a network segment. A /32 identifies a single, specific host IP address. All 32 bits are set in the mask (255.255.255.255), meaning there are no bits left for host addressing. There is no network address, no broadcast address, and no "subnet" in the traditional sense — only the host itself.

Full Breakdown

* A /32 has no network or broadcast address. The single IP is the host address itself.

Use Cases for /32

Loopback Interfaces on Routers and Switches

Network devices use /32 for loopback interfaces. A loopback is a virtual interface that is always up as long as the device is running — it is not tied to a physical port that could go down. Loopback IPs are used as:

• BGP router IDs — stable IP that identifies the router in BGP peering sessions
• OSPF router IDs — the highest loopback IP becomes the OSPF router ID by default
• Management IP — administrators SSH or SNMP poll the loopback IP because it stays reachable regardless of which physical interface is up

BGP Host Routes

ISPs and large networks advertise /32 host routes in BGP to announce a specific IP address to the internet. For example, an Anycast DNS server might be reached via a /32 route advertised from multiple locations. A /32 is also used in traffic engineering to steer specific IPs to specific paths.

AWS Security Group Rules

In AWS, when you add an inbound rule to a security group with a /32 CIDR source, you are allowing traffic from exactly one IP address:

• Adding 203.0.113.5/32 as an SSH source means only that IP can reach port 22
• This is the most restrictive and precise inbound rule — only one host can connect
• Commonly used to allow access from a specific admin workstation, CI/CD server, or partner IP

AWS Network ACLs and Firewall Rules

Network ACLs in AWS (and firewall rules generally) also use /32 to target a single IP address, for both allowing and denying traffic.

Static Host Routes

A static route with a /32 destination tells a router exactly where to send traffic destined for one specific IP — useful for routing management traffic, VPN split-tunneling configurations, or blackhole routes to block specific attackers.

Comparison: /32 vs /31 vs /30