IP Subnet Calculator: IPv4 CIDR, VLSM and IPv6 Prefixes

The IP subnet page is a three-panel CIDR workbench for addresses rather than connections. IPv4 Subnet takes one address and mask and fills ten result cards; VLSM splits a parent block between networks of different sizes; IPv6 normalises an address, reads its prefix and counts its addresses. Every field comes from bitwise arithmetic on the address itself, so the page works offline and sends nothing anywhere. For live measurements, the network tools page is the right surface.

Updated 2026-10-01Reviewed by CalcX Editorial Team5 sources4 min read

How to use the three panels

IPv4 Subnet — analyse an address.Type 192.168.1.130/26, 192.168.1.130 255.255.255.192, or a bare address with the mask in its own field. The cards fill as you type.
Prefix ↔ mask converter — switch notations.Enter 21, /21 or a dotted mask such as 255.255.248.0 for the prefix, dotted mask, wildcard and usable-host count.
VLSM — plan several networks.Give the parent as CIDR (192.168.1.0/24), then one subnet per line as name hosts (names optional, spaces allowed). The planner sorts by size and returns mask, range, broadcast and usable count per block.
IPv6 — read a prefix.Enter an address with an optional prefix (2001:db8:abcd:12::5/64): without one the text is expanded and compressed; with one you also get the network prefix, host bits and address counts.

Panels are also direct addresses: IPv4, VLSM and IPv6.

Worked examples

Every readout below was produced by the tool with the inputs shown.

A host inside a /26. 192.168.1.130/26 reads Network 192.168.1.128/26, Broadcast 192.168.1.191, Usable hosts 62, Total addresses 64, Host range 192.168.1.129 – 192.168.1.190, Netmask 255.255.255.192 = /26, Wildcard mask 0.0.0.63, Class C, Scope Private-use (RFC 1918) and role A host address.

A /24 split four ways. Parent 192.168.1.0/24 with A 100, B 50, C 25, D 5:

Name	Need	CIDR	Host range	Usable
A	100	192.168.1.0/25	192.168.1.1 – 192.168.1.126	126
B	50	192.168.1.128/26	192.168.1.129 – 192.168.1.190	62
C	25	192.168.1.192/27	192.168.1.193 – 192.168.1.222	30
D	5	192.168.1.224/29	192.168.1.225 – 192.168.1.230	6

Allocated / total 232 / 256, Utilisation 90.6 %, Next free 192.168.1.232.

A documentation prefix. 2001:db8:abcd:12::5/64 reads Compressed (RFC 5952) 2001:db8:abcd:12::5, Expanded 2001:0db8:abcd:0012:0000:0000:0000:0005, Network prefix 2001:db8:abcd:12::/64, Host bits 64 bit, Addresses exact 18446744073709551616 (magnitude 1.84×10^19) and Scope Documentation (RFC 3849).

How each panel computes

IPv4 fields. Network = address with host bits cleared; total addresses = 2^(32−prefix); broadcast = last address in the block; wildcard = inverted mask. Usable hosts follow 2^hostbits − 2, with the /31 and /32 exceptions below.
VLSM. Requests are sorted largest first and each gets the smallest power-of-two block that holds it; descending order keeps every block aligned, so the plan has no padding holes. Blocks are cut from the top and the summary reports what remains.
Scope. The address is matched against a built-in table of special-purpose ranges, longest prefix first. Only the three RFC 1918 blocks read Private-use; 100.64.0.0/10 (CGNAT) and 169.254.0.0/16 (link-local) are neither private nor publicly routable.
IPv6. Text is normalised to RFC 5952 form, and prefix arithmetic stays exact at 128 bits: address counts appear as a full decimal and as a magnitude.

Limits

Prefix ranges.: IPv4 prefixes run 0–32, and dotted masks must have contiguous 1 bits; 255.0.255.0 is rejected rather than quietly accepted.
Input syntax.: Octets with leading zeros (010.1.1.1) are declined because other parsers may read them as octal. A VLSM parent must be no narrower than /30, with positive integer host counts.
A built-in table, not a live registry.: Scope labels describe the address block, not whether a host is reachable.
Not a diagnostic or packet tool.: No DNS lookup, traceroute, speed test or public-IP check — that is the network tools page. No binary-register or colour work (programmer calculator), no decimal floating-point arithmetic (standard calculator).

The /31 trap

A /31 is not a wasted block. The formula 2^hostbits − 2 says a two-address prefix has zero usable hosts, but RFC 3021 point-to-point links use both: 192.168.1.4/31 reads Usable hosts 2 and Host range 192.168.1.4 – 192.168.1.5, with an info line noting there is no network or broadcast address. A /32 reads 1 as a host route; the VLSM planner keeps the −2 convention and never assigns /31 by itself.
Ties in IPv6 zero compression go to the leftmost run. 2001:db8:0:0:1:0:0:1 compresses to 2001:db8::1:0:0:1, not 2001:db8:0:0:1::1; a single zero group is never compressed.

Where it is useful

Splitting one block between teams

Needs of 100, 50, 25 and 5 hosts cannot share a /24 as equal /26s without wasting half the block. VLSM returns a /25, /26, /27 and /29, fills 232 of 256 addresses and leaves 192.168.1.232 free.

Reading an address from a ticket

A user reports 192.168.1.130/26; the panel shows the enclosing network, broadcast, host range and role — enough to tell a host from a gateway or broadcast address.

Checking a prefix before it ships

Paste a documentation address to confirm its canonical form, network prefix and exact address count before the prefix goes into a configuration file.

References

IETF, RFC 1918: Address Allocation for Private Internets, rfc-editor.org （访问日期：2026-10-01）— the private-use blocks behind the scope label.
IETF, RFC 4291: IP Version 6 Addressing Architecture, rfc-editor.org （访问日期：2026-10-01）— IPv6 structure, expanded form and special ranges.
IETF, RFC 4632: Classless Inter-domain Routing (CIDR), rfc-editor.org （访问日期：2026-10-01）— prefix notation and the network/host split.
IETF, RFC 5952: A Recommendation for IPv6 Address Text Representation, rfc-editor.org （访问日期：2026-10-01）— zero compression and the tie rule.
IETF, RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links, rfc-editor.org （访问日期：2026-10-01）— why both /31 addresses are usable.