Single Host: /32 (1 Address)
Learn about /32 CIDR blocks that represent a single IP address. Covers use cases in routing, security groups, ACLs, and host-specific firewall rules.
Detailed Explanation
/32: A Single IP Address
A /32 CIDR block represents exactly one IP address. While it may seem trivial, /32 routes and rules are fundamental to networking, security, and traffic engineering.
Range Details (Example: 10.0.1.42/32)
| Property | Value |
|---|---|
| Network Address | 10.0.1.42 |
| Broadcast Address | 10.0.1.42 |
| Subnet Mask | 255.255.255.255 |
| Wildcard Mask | 0.0.0.0 |
| Total Addresses | 1 |
| Usable Hosts | 1 |
Common Uses for /32
1. Security Group / Firewall Rules
Allow SSH only from your specific IP:
Inbound Rule: TCP 22 from 203.0.113.42/32
2. Host Routes in Routing Tables
Force traffic for a specific host through a particular gateway:
ip route add 10.0.1.42/32 via 10.0.0.1
3. BGP Announcements
ISPs use /32 routes for blackholing attack traffic or traffic engineering specific destination IPs.
4. Loopback Addresses
Network devices (routers, switches) assign /32 addresses to their loopback interfaces as stable identifiers:
interface Loopback0
ip address 10.255.0.1 255.255.255.255
5. Elastic IPs / Floating IPs
Cloud providers internally route Elastic IPs (AWS) or Floating IPs (OpenStack) as /32 routes to the correct instance.
/32 in CIDR Overlap Checking
When checking overlaps, a /32 is contained by any CIDR block that includes its address. For example, 10.0.1.42/32 is contained within 10.0.1.0/24, 10.0.0.0/16, and 10.0.0.0/8.
Use Case
Writing security group rules that restrict access to a single IP address, configuring host-specific routes in a routing table, or setting up loopback addresses on network devices for OSPF/BGP router IDs.