Usable Hosts Calculator

Calculator Inputs

The page stays in a single-column flow, while the form uses a responsive 3-column, 2-column, and 1-column grid.

Allocation Label

IPv4 Address

CIDR Prefix

Required Hosts

Reserve Percentage

Classful Base

Treat /31 as usable point-to-point addressing

Reset

Formula Used

Total addresses:
Total Addresses = 2^{(32 − Prefix)}

Classic usable hosts:
Usable Hosts = Total Addresses − 2

Special cases:
/32 gives one host route. /31 can give two usable endpoints when RFC 3021 point-to-point mode is enabled.

Network address:
Network = IP Address AND Subnet Mask

Broadcast address:
Broadcast = Network Address OR Wildcard Mask

Wildcard mask:
Wildcard Mask = 255.255.255.255 − Subnet Mask

First and last usable:
First Usable = Network + 1, Last Usable = Broadcast − 1, except special /31 and /32 handling.

Planned host requirement:
Planned Hosts = ceil(Required Hosts × (1 + Reserve Percentage / 100))

How to Use This Calculator

Enter the IPv4 address you want to evaluate.
Type the CIDR prefix from 0 to 32.
Add required hosts for capacity planning.
Enter a reserve percentage for future growth.
Keep classful base on auto, or force A, B, or C.
Enable /31 mode when designing point-to-point links.
Press the calculate button to generate network details.
Review network, broadcast, usable range, masks, and utilization.
Use the CSV or PDF buttons to export results.
Study the graph to compare capacity, usage, and planned growth.

Example Data Table

IPv4 Address	Prefix	Network	Broadcast	Total Addresses	Usable Hosts	First Usable	Last Usable
192.168.10.34	/24	192.168.10.0	192.168.10.255	256	254	192.168.10.1	192.168.10.254
10.25.14.200	/20	10.25.0.0	10.25.15.255	4,096	4,094	10.25.0.1	10.25.15.254
172.16.5.129	/26	172.16.5.128	172.16.5.191	64	62	172.16.5.129	172.16.5.190
203.0.113.77	/30	203.0.113.76	203.0.113.79	4	2	203.0.113.77	203.0.113.78
198.51.100.9	/29	198.51.100.8	198.51.100.15	8	6	198.51.100.9	198.51.100.14

FAQs

1) What does usable hosts mean?

Usable hosts are the addresses you can assign to devices inside a subnet. In classic IPv4 subnetting, the network and broadcast addresses are reserved and cannot be assigned to normal hosts.

2) Why does a subnet lose two addresses?

Most IPv4 subnets reserve one address for the network identifier and one for the broadcast address. That is why usable host count is usually total addresses minus two.

3) Why is /31 treated differently sometimes?

A /31 subnet is commonly used on point-to-point links. RFC 3021 allows both addresses to be used as endpoints, so there is no separate broadcast address in that design mode.

4) What happens with a /32 prefix?

A /32 identifies one exact host route. It has one address only, so the calculator reports that single address as the host route rather than a normal subnet range.

5) Why include reserve percentage?

Reserve percentage helps you plan future growth. Instead of sizing a subnet only for current devices, you can include expansion margin and choose a prefix that lasts longer.

6) What is the wildcard mask used for?

Wildcard masks are often used in routing, ACLs, and network matching rules. They represent the inverse of the subnet mask and show which bits can vary.

7) Why does the calculator show class and classful base?

Classful information is useful for learning subnetting and estimating borrowed bits or subnet counts from traditional A, B, and C network boundaries. Modern routing still uses classless CIDR.

8) When should I use the recommended prefix result?

Use the recommended prefix when you know how many hosts you need. It gives the smallest subnet that can handle required devices plus your chosen reserve margin.