/31 vs /32 — Subnet Comparison
A /31 subnet is 2× larger than a /32. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /31 has 21 = 2 times as many addresses.
2 host addresses — RFC 3021 P2P
Typical Uses
- →Point-to-point link (more efficient than /30)
- →Router interface addressing with RFC 3021 support
- →BGP peering links
Single host address
Typical Uses
- →Host route in routing table
- →Loopback interface address
- →Security group rule for a single IP
Key Differences
How 2 /32 Subnets Divide a /31
Example using 10.0.0.0/31 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/32 | 10.0.0.0 | 10.0.0.0 | 10.0.0.0 | 10.0.0.0 | 1 |
| 2 | 10.0.0.1/32 | 10.0.0.1 | 10.0.0.1 | 10.0.0.1 | 10.0.0.1 | 1 |
FAQ
What is the difference between /31 and /32?
A /31 has 2 usable hosts
and a /32 has 1.
The subnet masks differ: /31 uses 255.255.255.254
while /32 uses 255.255.255.255.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/31 is exactly 2× larger.
How many /32 subnets fit in a /31?
Exactly 2 /32 subnets fit perfectly inside one /31 with no wasted space. To split a /31 into /32s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
/31 is typically used for: Modern P2P links (RFC 3021). /32 is better for: Single host — firewall rules, host routes. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.