IP Subnet Calculator (IPv4)

Network Summary

{{ maskDotted }} mask {{ totalAddresses }} total {{ usableHosts }} usable {{ networkBitsBadge }} {{ hostBitsBadge }} {{ ipClass }} class {{ rfcLabel }}

IP address:

Prefix:

Host index (0-based):

Show first N hosts:

Supernet levels:

Field	Value	Copy
{{ row.key }}	{{ row.value }}

Index	Address	Type	Copy
{{ h.index }}	{{ h.address }}	{{ h.kind }}

Which	CIDR	Network	Broadcast	Copy
{{ r.which }}	{{ r.cidr }}	{{ r.network }}	{{ r.broadcast \|\| '—' }}
No neighbors at this boundary.

Level	CIDR	Network	Broadcast	Usable	Copy
{{ u.level }}	{{ u.cidr }}	{{ u.network }}	{{ u.broadcast \|\| '—' }}	{{ u.usable }}
Increase supernet levels to aggregate upward.

Prefix	Mask	Wildcard	Total addresses	Usable hosts	Host bits	Copy
/{{ row.prefix }}	{{ row.mask }}	{{ row.wildcard }}	{{ row.total }}	{{ row.usable }}	{{ row.hostBits }}

No ladder data yet. Calculate a subnet to view nearby prefixes.

Field	Decimal	Binary	Copy
{{ row.key }}	{{ row.dec }}	{{ row.bin }}

Tags: IP Addressing , Networking , Sysadmin

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IPv4 addresses are numbered locations that identify devices on a network and group them into subnets for routing and access control. A clear view of the network address, the usable host range, and the broadcast behavior helps you size blocks, avoid collisions, and plan growth. Many readers search for an IPv4 subnet calculator with CIDR prefix support when they need a quick, reliable answer without guesswork.

Enter an address and a prefix so the subnet can be derived, then read the network boundary, usable hosts, and any special range label. You can look up one host by its index, inspect binary representations for teaching or audits, compare adjacent networks to place the next block correctly, and open the CIDR tab to compare nearby prefixes with mask, wildcard, and host-bit counts.

A typical example is a small office where 192.168.1.10 with a prefix of 24 yields a network boundary at 192.168.1.0 and a usable range ending at 192.168.1.254 so printers and servers can be assigned predictable addresses. Results reflect point to point behavior for a prefix of 31 and single address behavior for a prefix of 32 so edge cases remain accurate.

For consistent results, keep inputs in dotted decimal, confirm the intended prefix, and prefer measured host counts over mental math. When moving from a lab to production, repeat the calculation and record the network and broadcast addresses in change notes.

Technical Details:

The quantities of interest are the IPv4 address (four octets), the Classless Inter‑Domain Routing (CIDR) prefix length, the derived subnet mask, and the resulting addresses for network, broadcast, first host, and last host. The computation treats the address as a 32 bit unsigned value, producing a snapshot of one subnet rather than a time series.

Total capacity is computed from the prefix, then mapped to usable host count with special handling for very small subnets. Reading these values tells you how many interfaces can be numbered and where the usable range starts and ends. A class label is derived from the first octet and a special range label is applied when the address falls within private, shared, loopback, link local, multicast, reserved, or test allocations exposed by the code. Mask and wildcard values are mirrored in hexadecimal, and the computed network and host bit counts surface how much of the 32-bit space remains for addressing.

Interpretation follows common subnetting practice. For prefix lengths of 31 the two addresses are both usable and no broadcast is reported. For prefix lengths of 32 the single address is the only usable value. For larger blocks, the first and last usable hosts exclude the network and broadcast endpoints.

Comparisons are valid for a fixed prefix and address family. The class label is descriptive and should not be used to infer routing policy. Large ranges list only a sample of hosts to keep tables responsive.

N_total = 2^{32 - p}

\begin{array}{lll} n & = & ip \land m \\ b & = & n \lor \neg m \end{array}

\begin{array}{lll} N_usable & = & 1 if p = 32 \\ 2 if p = 31 \\ N_total - 2 otherwise \end{array}

Symbols and units
Symbol	Meaning	Unit/Datatype	Source
ip	IPv4 address	dotted string	Input
p	CIDR prefix length	integer	Input
m	Subnet mask	dotted string	Derived
w	Wildcard mask	dotted string	Derived
n	Network address	dotted string	Derived
b	Broadcast address	dotted string	Derived
N_total	Total addresses	count	Derived
N_usable	Usable hosts	count	Derived

Worked example. Input 192.168.1.10 with prefix 24. Mask 255.255.255.0. Network 192.168.1.0. Broadcast 192.168.1.255. Total addresses 256. Usable hosts 254. First host 192.168.1.1. Last host 192.168.1.254. Class C. Special label RFC1918 Private (192.168/16).

Validation and bounds derived from the interface
Field	Type	Min	Max	Step/Pattern	Error Text	Placeholder
IP address	text	—	—	IPv4 or IPv4 with prefix or “IP mask” pair	“Invalid IPv4 address.” or “Invalid CIDR prefix.”	192.168.1.10 or 192.168.1.0/24
Prefix	select	0	32	integer	“Invalid CIDR prefix.”	/p with dotted mask
Host index	number	0	usable − 1 (or 1 for 31)	integer	Out‑of‑range returns empty	e.g. 5
Show first N hosts	select	16	2048	choices plus “All”	—	All or 16 to 2048
Supernet levels	number	0	8	integer	—	—

Special range labels applied by the logic include RFC1918 Private (10/8, 172.16/12, 192.168/16), RFC6598 Shared (100.64/10), Loopback (127/8), Link‑local (169.254/16), TEST‑NET‑1/2/3, Benchmark (198.18/15), IETF Protocol Assignments (192.0.0/24), Multicast (224/4), Reserved (240/4), and This network (0/8).

Class labels by first octet
Class	Lower	Upper	Interpretation	Action Cue
A	0	127	Historical unicast label	Large legacy blocks
B	128	191	Historical unicast label	Medium legacy blocks
C	192	223	Historical unicast label	Common small blocks
D	224	239	Multicast	Not for hosts
E	240	255	Reserved	Not for hosts

I/O summary
Input	Accepted Families	Output	Encoding/Precision	Rounding
IPv4 address, prefix, options	IPv4 dotted decimal	Tables, CSV copy/download, JSON view/download	Exact integers, dotted strings	Not applicable

Networking and storage behavior: all calculations run on the page and copy or download actions operate locally.

Performance: computations are constant time per address, while host listing is linear in the requested count and capped by the selector to remain responsive.

Diagnostics and determinism: identical inputs produce identical results; errors surface as concise messages above the results panel.

Security considerations: inputs are parsed as plain text and rendered into tables and preformatted blocks; no secrets or credentials are required.

Assumptions & limitations

IPv4 only; IPv6 is not supported.
Non contiguous masks are rejected when converting to a prefix. Heads‑up
Class label is descriptive and does not imply routing policy.
“All” host listing can be very large on small prefixes. Heads‑up
Neighbor networks are computed at the same prefix only.
Supernet aggregation is limited to eight levels.
First and last host skip network and broadcast only when capacity exceeds two.
Unsigned 32 bit arithmetic is used for all bitwise operations.

Edge cases & error sources

Octets outside 0 to 255 or non integers.
Missing octets or extra separators.
Prefix outside 0 to 32.
Mask that is not contiguous when parsed from “IP mask”.
Host index negative or beyond the usable range.
“All” on a very large network may stall the page.
No previous neighbor below 0.0.0.0 and no next neighbor beyond 255.255.255.255.
Locale formatting of large counts may vary in chart labels.
Clipboard permissions can block copy actions.
Slow devices may struggle with long host tables.

Step‑by‑Step Guide

IPv4 subnet analysis produces the network boundary, usable range, and optional neighbors and supernets.

Enter an address in dotted decimal IP address.
Select or confirm the Prefix.
Expand advanced options to set a Host index or host list size.
Optionally choose Supernet levels to aggregate upward.
Read the “Info” tab for network, counts, class, and special label.
Use “CIDR”, “Neighbors”, “Supernets”, “Binary”, “Hosts”, and “JSON” as needed.

Example. Input 10.0.5.12 with prefix 20 returns network 10.0.0.0 and last host 10.0.15.254 to cover 4094 usable addresses in that block.

Copy any table or download as CSV for hand‑off.
Switch to “Binary” to teach bitwise boundaries.
Check “Neighbors” before placing a new block.

FAQ

Is my data stored?

No. Calculations run locally and copy or download actions originate from the page.

No server requests are made.

How accurate is the host count?

Counts follow exact integer arithmetic. Prefix 31 yields two usable addresses. Prefix 32 yields one usable address. Larger blocks exclude network and broadcast.

What formats can I enter?

Use IPv4, IPv4 with a CIDR prefix, or an address followed by a dotted mask separated by space.

Can I list every host?

Yes for small blocks. For large blocks the “All” option may be too heavy; select a capped size like 256 or 2048 to stay responsive.

How do I find the next /24 boundary?

Set the prefix to 24 and open “Neighbors” to read the next network and its broadcast address. Place the following block at the next network.

What does a “borderline” result mean?

If the prefix is 31 or 32, the usual network and broadcast rules change. Both addresses are usable at 31 and a single address exists at 32.

Does it support IPv6?

No. The logic and display target IPv4 only.

Are special ranges identified?

Yes. Private, shared, loopback, link local, test, multicast, reserved, and related blocks are labeled when the input falls within them.

Troubleshooting

“Invalid IPv4 address.” — Check all four octets are 0 to 255.
“Invalid CIDR prefix.” — Use an integer from 0 to 32.
Wrong mask from “IP mask” — Ensure the mask is contiguous.
No “Host at index” — Adjust the index within the usable range.
Slow table — Reduce “Show first N hosts”.
Empty neighbors — The current network is at an address boundary.
Copy fails — Allow clipboard permissions or use download.

Selecting “All” for a very large block can exhaust memory. Choose a smaller host sample.

Glossary

CIDR prefix: Number of leading mask bits that define the subnet.
Subnet mask: Dotted decimal mask derived from the prefix.
Wildcard mask: Bitwise inverse of the subnet mask.
Network address: The base address of the subnet after masking.
Broadcast address: The final address in the subnet for broadcasts.
Host index: Zero based offset from the first usable host.
Supernet: An aggregate block formed by reducing the prefix.
Neighbor networks: The previous and next blocks at the same prefix.

IP Subnet Calculator (IPv4)

Network Summary

Introduction:

Technical Details:

Assumptions & limitations

Edge cases & error sources

Step‑by‑Step Guide

FAQ

Troubleshooting

Glossary