Calculator Inputs
Example Data Table
| Entered IP | CIDR | Total Addresses | Usable Hosts | First Usable | Last Usable | Broadcast |
|---|---|---|---|---|---|---|
| 192.168.10.25 | /24 | 256 | 254 | 192.168.10.1 | 192.168.10.254 | 192.168.10.255 |
| 10.20.14.200 | /27 | 32 | 30 | 10.20.14.193 | 10.20.14.222 | 10.20.14.223 |
| 172.16.4.9 | /29 | 8 | 6 | 172.16.4.9 | 172.16.4.14 | 172.16.4.15 |
Formula Used
Total addresses = 2(32 − prefix)
Standard usable hosts = Total addresses − 2
/31 usable hosts = 2, because point-to-point links can use both addresses.
/32 usable hosts = 1, because the route identifies one host only.
Remaining usable hosts = max(Usable hosts − Reserved hosts, 0)
Buffered host need = ceil(Required hosts × (1 + Growth buffer / 100))
Effective host need = Buffered host need + Reserved hosts
Recommended prefix = the smallest subnet that still provides usable hosts greater than or equal to the effective need.
How to Use This Calculator
- Enter an IPv4 address that belongs to the subnet you want to analyze.
- Provide the CIDR prefix, such as 24 for a /24 network.
- Add reserved host slots for gateways, load balancers, monitoring devices, or future infrastructure.
- Optionally enter required usable hosts and a growth buffer to test whether the current subnet is large enough.
- Press the calculate button to see the network address, broadcast address, usable range, binary masks, and capacity metrics.
- Use the CSV or PDF buttons to export the current result set.
Frequently Asked Questions
1) What is a usable IP address?
A usable IP address is one that can be assigned to a host interface inside a subnet. In standard IPv4 subnetting, the network and broadcast addresses are not counted as normal host addresses.
2) Why do many subnets lose two addresses?
Most IPv4 subnets reserve the first address as the network identifier and the last address as the broadcast address. That leaves total addresses minus two for ordinary hosts.
3) Does a /31 really provide two usable addresses?
Yes. On point-to-point links, /31 networks commonly treat both addresses as usable. That behavior reduces waste and is widely used for router-to-router connections.
4) What should I enter as reserved hosts?
Use reserved hosts for addresses you do not want available to general devices. Common examples include gateways, firewalls, VPN endpoints, printers, controllers, and future infrastructure reservations.
5) Why might my current subnet fail the requirement check?
The subnet may not have enough usable capacity after subtracting reserved space and adding the growth buffer. A tighter prefix gives more addresses, while a larger prefix value gives fewer.
6) Do private and public IPv4 ranges change the math?
No. Private or public status affects routing and exposure, but the address math is the same. The same prefix always yields the same total and usable address counts.
7) Why is a growth buffer useful in subnet planning?
A growth buffer prevents immediate readdressing when more devices appear later. It helps size a subnet for future users, appliances, services, and infrastructure without overcommitting too early.
8) Can this calculator help with VLAN planning?
Yes. It is useful for estimating host capacity per VLAN, validating subnet sizes, and checking whether a planned segment can support required devices plus infrastructure and growth.