IP Subnet Calculator

How to Use the IP Subnet Calculator

Use the main fields for the quickest subnet lookup, then open Advanced Options only when you need host sizing, equal subnet splitting, or a same-subnet comparison.

1. Choose IPv4 or IPv6

   Select IPv4 for dotted-decimal addresses such as 192.168.1.100 or IPv6 for colon-separated hexadecimal addresses such as 2001:db8::1. The prefix limits and result conventions update with the selected IP version.

2. Enter the address

   You can enter the address by itself or include the prefix in the same field, such as 192.168.1.100/24. For IPv6, compressed notation such as 2001:db8::1/64 is accepted.

3. Enter the prefix or IPv4 subnet mask

   For IPv4, enter a CIDR prefix from /0 through /32 or a contiguous dotted mask such as 255.255.255.0. For IPv6, enter a prefix from /0 through /128.

4. Read the subnet result

   The result updates automatically when the active inputs are valid. Start with the network address or network prefix, then check the address range, mask details, address capacity, and address classification.

5. Use Advanced Options when needed

   Required Usable Hosts finds the smallest conventional IPv4 subnet that meets a requested host count. Split Into Prefix divides the current network into equal child subnets. Second IP Address checks whether another address resolves to the same network under the current prefix.

What the Inputs and Results Mean

Subnet calculations are exact once the IP address and prefix are defined. Most mistakes come from entering the wrong prefix, confusing a host address with a network address, or applying IPv4 conventions to IPv6.

IP Address

The address being located inside a subnet. It does not need to be the network address; any valid host or endpoint address within the block can be used.

CIDR Prefix

The number of leading address bits assigned to the network portion. IPv4 uses /0 to /32; IPv6 uses /0 to /128.

Subnet Mask

The IPv4 dotted-decimal representation of the same prefix boundary. For example, /24 corresponds to 255.255.255.0 and /26 corresponds to 255.255.255.192.

Network Address / Network Prefix

The start of the calculated address block after host bits are cleared to zero. This is the calculator’s primary result.

Broadcast Address

For conventional IPv4 subnets, the all-ones host-bit address at the top of the block. IPv6 does not define broadcast addresses; multicast performs that function instead.

Usable Host Range

For conventional IPv4 subnets, the range between network and broadcast addresses. /31 and /32 require special interpretation and are handled separately by the calculator.

Wildcard Mask

The bitwise inverse of the IPv4 subnet mask. A /26 mask of 255.255.255.192 has wildcard mask 0.0.0.63.

Total Addresses

The exact size of the CIDR block. If \(h\) host bits remain, the block contains \(2^h\) address values.

Usable Hosts

The calculator reports conventional IPv4 host capacity, with explicit special handling for /31 and /32. IPv6 instead reports total address values in the prefix rather than applying IPv4’s network/broadcast subtraction.

Address Type

The tool identifies several common special-use categories, including RFC 1918 private IPv4 space, loopback, link-local, shared/CGNAT, documentation, multicast, and selected IPv6 special-use categories.

How IP Subnet Calculation Works

CIDR makes the network boundary explicit. For IPv4, a prefix \(p\) marks the first \(p\) of 32 bits as network bits; IPv6 applies the same idea to 128 bits. The network is found by keeping those prefix bits and clearing the remaining host bits.

How to convert CIDR to an IPv4 subnet mask

A CIDR prefix is the number of leading 1 bits in the subnet mask. For /26, write 26 ones followed by 6 zeros:

Convert each 8-bit octet to decimal: 11111111 is 255 and 11000000 is 192, giving 255.255.255.192. To convert a dotted mask back to CIDR, count the contiguous 1 bits. A valid CIDR mask cannot switch back to 1 after a 0 appears.

How to calculate the IPv4 wildcard mask

The wildcard mask is the bitwise inverse of the subnet mask. In dotted decimal, subtract each mask octet from 255:

For /26, 255.255.255.255 − 255.255.255.192 = 0.0.0.63. Wildcard masks are commonly used when matching address ranges in network access-control and routing configurations.

\(p\)

Prefix length Number of leading bits assigned to the network prefix. bits

\(B\)

Address width Total number of bits in the selected IP version. 32 bits IPv4; 128 bits IPv6

\(h\)

Host or interface bits Bits remaining after the network prefix is reserved. bitsderived value

\(N\)

Total address values Exact number of addresses represented by the prefix. count

\(H\)

Conventional usable IPv4 hosts Host capacity after excluding network and broadcast positions for ordinary IPv4 subnets through /30. count

Worked Example: 192.168.1.100/26

Suppose the entered address is 192.168.1.100 with prefix /26. The goal is to find the network, broadcast address, usable host range, and capacity.

/24 split into four /26 subnets

How to Interpret and Check a Subnet Result

A correct subnet result tells you where the address block begins, where it ends, how large it is, and—under IPv4 conventions—which addresses are normally assignable to hosts. The most useful sanity check is that the entered address must fall inside the returned network range.

How the same-subnet check works

Two addresses are in the same subnet when applying the same prefix produces the same network address. For example, 192.168.1.20/24 and 192.168.1.200/24 both resolve to 192.168.1.0/24. Change the second address to 192.168.2.20/24 and its network becomes 192.168.2.0/24, so the addresses are not in the same subnet.

Choosing a subnet from a host requirement

The calculator’s Required Usable Hosts option is deliberately a conventional IPv4 sizing check. It searches from /30 downward and returns the longest prefix whose \(2^h-2\) capacity still meets the requested host count. For 50 hosts, /27 is too small because it provides 30 usable hosts, while /26 provides 62, so /26 is the smallest conventional fit.

CIDR, Host Capacity, and Private-Range Reference

Use these tables to convert IPv4 CIDR prefixes, estimate conventional host capacity, and recognize the three RFC 1918 private ranges. The full CIDR table shows arithmetic block capacity; it does not imply that every address in a very large block is globally assignable or routable.

Common IPv4 prefixes at a glance

Full IPv4 /0–/32 CIDR table

CIDR block sizes follow the classless prefix model described in RFC 4632. The /31 row follows the point-to-point behavior in RFC 3021; /32 denotes a single-address host route in the CIDR size table.

Smallest conventional IPv4 subnet by required hosts

The sizing table mirrors the calculator’s conventional Required Usable Hosts logic, which does not choose /31 or /32 for ordinary host-capacity sizing. Those prefixes have special roles handled separately.

RFC 1918 private IPv4 ranges

RFC 1918 reserves exactly these three blocks for private internets. Addresses outside them are not automatically ordinary public addresses: IANA also maintains a special-purpose IPv4 registry for ranges such as shared address space, loopback, link-local, documentation, and protocol-specific blocks.

Common Subnetting Mistakes

Most subnetting errors are not arithmetic errors; they are interpretation errors. The prefix must control the decision, not familiar-looking decimal endings or old classful assumptions.

IPv4 and IPv6 Edge Cases and Limits

The subnet math is deterministic, but the meaning of certain prefixes depends on protocol conventions. The calculator handles the major boundary cases explicitly instead of forcing one host-count rule onto every network.

IPv4 vs. IPv6 subnetting

IPv6 worked example: 2001:db8:abcd:1234::1/64

With a /64 prefix, the first 64 bits identify the network and 64 bits remain for interface addressing. The network prefix is 2001:db8:abcd:1234::/64.

That number is the count of address values in the prefix, not an IPv4-style “usable host” count. IPv6 does not reserve a broadcast address, so the calculator correctly reports broadcast as not applicable.

CIDR aggregation and route summarization

CIDR can also summarize aligned, contiguous networks. For example, 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24 share the same first 22 bits and can be represented by 192.168.0.0/22. Aggregation only works when the combined block is correctly aligned and covers a contiguous power-of-two address range.

Sources and Calculation Verification

The calculator and this guide use exact prefix arithmetic, with protocol-specific edge cases checked against the relevant IETF RFCs and IANA address registries.

• RFC 4632 — Classless Inter-domain Routing (CIDR) — supports 32-bit IPv4 CIDR prefix notation, power-of-two block sizes, /31 point-to-point notation, /32 host-route notation, and classless addressing.
• RFC 3021 — Using 31-Bit Prefixes on IPv4 Point-to-Point Links — supports interpreting the two /31 address values as host endpoints on qualifying point-to-point links.
• RFC 4291 — IP Version 6 Addressing Architecture — supports 128-bit IPv6 addressing and the absence of broadcast addresses in IPv6.
• RFC 6164 — Using 127-Bit IPv6 Prefixes on Inter-Router Links — supports the /127 point-to-point IPv6 special case.
• RFC 1918 — Address Allocation for Private Internets — defines the three private IPv4 blocks 10/8, 172.16/12, and 192.168/16.
• IANA IPv4 Special-Purpose Address Space — provides the current registry for private-use, shared, loopback, link-local, documentation, and other special-purpose IPv4 blocks.

The worked /26 example was checked two independent ways: by the mask/block-size shortcut and by binary bitwise AND. Both produce network 192.168.1.64, broadcast 192.168.1.127, and 62 conventional usable hosts. The article’s interface descriptions were matched against the finalized IP Subnet Calculator HTML and JavaScript used on this page.

IP Subnet Calculator FAQ

These answers cover common subnet questions that are easy to misinterpret even after the arithmetic is correct.