Azure Subnet Calculator
CIDR calculator that knows Azure's rules: the 5 reserved addresses per subnet, real usable host counts, and subnet splitting.
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total addresses
251
usable in Azure
254
usable elsewhere
| Network | 10.0.1.0/24 (mask 255.255.255.0) |
|---|---|
| Azure-usable range | 10.0.1.4 – 10.0.1.254 |
| Next subnet (same size) | 10.0.2.0/24 |
The 5 reserved addresses
| 10.0.1.0 | Network address |
|---|---|
| 10.0.1.1 | Azure: default gateway |
| 10.0.1.2 | Azure: DNS mapping |
| 10.0.1.3 | Azure: DNS mapping |
| 10.0.1.255 | Broadcast address (reserved by Azure) |
Frequently asked
Why are 5 addresses reserved?
Azure keeps the network address, broadcast address, and three platform addresses (gateway + two DNS) in every subnet — so a /24 gives 251 usable, not 254.
What's the smallest Azure subnet?
/29 (8 addresses, 3 usable). Smaller prefixes are rejected.