Last reviewed: May 2026
Both /30 and /31 are common choices, and the right one depends less on host counts than on where the block sits in your wider plan. Here is the direct comparison, then the practical guidance.
| Prefix | Total IPs | Usable (standard) | AWS usable | Azure usable | GCP usable | Typical use |
|---|---|---|---|---|---|---|
| /30 | 4 | 2 | n/a | n/a | n/a | Classic point-to-point link |
| /31 | 2 | 2 | n/a | n/a | n/a | Modern point-to-point link (RFC 3021) |
AWS and Azure reserve 5 addresses per subnet; GCP reserves 4. Those columns are what you can actually assign to workloads.
Legacy equipment, or any device whose firmware predates RFC 3021 support. A /30 wastes two of its four addresses on network and broadcast, but it is universally understood and will never surprise you on an old platform.
Modern router-to-router and firewall-to-firewall links. RFC 3021 allows both addresses in a /31 to be used as host addresses, because a point-to-point link has no need for a broadcast address. Across a large WAN this halves the address consumption of your link estate.
Prefix choice is a downstream decision. Start from the VPC or VNet block, divide it per region and per availability zone, and only then choose subnet sizes. The Cloud VPC & VNet CIDR Planning Guide covers that top-down process, and the free subnet finder will tell you which of these sizes actually fits in the space you have left.
Neither is better in the abstract. A /30 has more usable addresses and a /31 conserves address space. Choose based on expected host count, provider minimums for the service involved, and how much room the parent block has left.
Exactly 2, because the prefix difference is 1 bit and 2^1 = 2.
Both reserve five addresses in every subnet: the network address, the default gateway, two for internal DNS and future use, and the broadcast address. GCP reserves four. A /30 therefore yields fewer assignable addresses in cloud than the textbook figure.