/30 vs /31 — Subnet Comparison
A /30 subnet is 2× larger than a /31. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /30 has 21 = 2 times as many addresses.
2 usable hosts — point-to-point link
Typical Uses
- →WAN point-to-point link between routers
- →BGP peering session
- →Dedicated leased line addressing
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
Key Differences
How 2 /31 Subnets Divide a /30
Example using 10.0.0.0/30 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/31 | 10.0.0.0 | 10.0.0.0 | 10.0.0.1 | 10.0.0.1 | 2 |
| 2 | 10.0.0.2/31 | 10.0.0.2 | 10.0.0.2 | 10.0.0.3 | 10.0.0.3 | 2 |
FAQ
What is the difference between /30 and /31?
A /30 has 2 usable hosts
and a /31 has 2.
The subnet masks differ: /30 uses 255.255.255.252
while /31 uses 255.255.255.254.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/30 is exactly 2× larger.
How many /31 subnets fit in a /30?
Exactly 2 /31 subnets fit perfectly inside one /30 with no wasted space. To split a /30 into /31s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
/30 is typically used for: WAN point-to-point link. /31 is better for: Modern P2P links (RFC 3021). Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.