Developer Tools
IPv6 Subnet Calculator
Calculate IPv6 prefixes in your browser. Network and last address, total host count, /64 subnet count, reverse DNS, EUI-64, split a parent into children.
IPv6 subnet calculator
Mode
Calculate a single prefix, split a parent prefix into equal-sized children, or open the prefix-length reference table.
Accepts compressed (::), full 8-group, embedded IPv4, optional %zone, and optional /prefix. Use the prefix override below if the address has no slash.
Leave blank to use the /prefix on the address (default 64 when none is provided).
Quick samples
Network summary
Network (CIDR, compressed)
2001:db8:cafe:1::/64
RFC 5952 short form. Drop into Linux ip, iproute2, FRR, BIRD, AWS, GCP, and most APIs.
Network (CIDR, expanded)
2001:0db8:cafe:0001:0000:0000:0000:0000/64
Every group padded to 4 hex digits. Used by older Cisco IOS and some logs.
First address
2001:db8:cafe:1::
All host bits zero. The network address itself.
Last address
2001:db8:cafe:1:ffff:ffff:ffff:ffff
All host bits one. No broadcast address exists in IPv6, so this is also a usable host.
Sizing
Prefix length
/64
Network bits: 64. Host bits: 64.
Total addresses
2^64 approx 1.84 × 10^19
Power-of-two form shown because the exact integer is too large for a single line.
/64 subnets contained
1 (1)
Each /64 is one LAN. RFC 6177 recommends sites use /48 with up to 65,536 LANs.
Reverse DNS (ip6.arpa)
0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0.e.f.a.c.8.b.d.0.1.0.0.2.ip6.arpa
PTR record stem for the network address. Append host nibbles for individual records.
Classification
Scope: Global unicast
- Documentation (2001:db8::/32, RFC 3849). Reserved for examples and docs.
- Global unicast (2000::/3). Routable on the public IPv6 internet.
Modified EUI-64 from a MAC address (RFC 4291)
Generates the bottom 64 bits (interface identifier) by inserting ff:fe in the middle of the MAC and flipping the universal/local bit. Uses the network prefix above as the top 64 bits.
EUI-64 interface identifier
021a:2bff:fe3c:4d5e
Bottom 64 bits of the SLAAC address.
Full address (network prefix + EUI-64 IID)
2001:db8:cafe:1:21a:2bff:fe3c:4d5e
The address a stateless host with this MAC would auto-configure (modern OSes prefer privacy addresses, RFC 8981).
Binary breakdown
0010000000000001 0000110110111000 1100101011111110 0000000000000001 0000000000000000 0000000000000000 0000000000000000 0000000000000000
128 bits, eight 16-bit groups, network address. The first /64 bits identify the network; the remaining 64 bits identify the host.
Mask (hextet form)
ffff:ffff:ffff:ffff:0000:0000:0000:0000
/64 expressed as an eight-group hextet mask. Useful for tooling that does not accept a prefix length.
Subnet identifier (SID)
0x1
The 16-bit group between bits 48 and 64. Identifies one LAN inside an end-site /48.
Report
Copy a clean plain-text summary for a change ticket, pull request, doc, or note.
Input: 2001:db8:cafe:1::/64 Network (compressed): 2001:db8:cafe:1::/64 Network (expanded): 2001:0db8:cafe:0001:0000:0000:0000:0000/64 First address: 2001:db8:cafe:1:: Last address: 2001:db8:cafe:1:ffff:ffff:ffff:ffff Reverse DNS: 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0.e.f.a.c.8.b.d.0.1.0.0.2.ip6.arpa Prefix length: /64 Host bits: 64 Total addresses: 2^64 (2^64) /64 subnets in this prefix: 1 (1) Scope: Global unicast - Documentation (2001:db8::/32, RFC 3849). Reserved for examples and docs. - Global unicast (2000::/3). Routable on the public IPv6 internet.
How to use
- Pick Calculate to size a single prefix, Split to divide a parent prefix into equal-sized children, or Reference to read the prefix-length table.
- In Calculate, paste an IPv6 address or prefix (2001:db8:cafe:1::/64 works; so does 2001:db8::1 with a /64 in the prefix override field).
- Read the network and last address, the prefix length and host bits, the total address count (exact or as a power of two), the /64 subnet count when applicable, and the reverse DNS stem.
- Read the Classification panel for the scope (Global Unicast, ULA, Link-local, Multicast, Documentation, and other reserved ranges).
- For a /64 or shorter prefix, paste a MAC address in the EUI-64 panel to derive the interface identifier and the full SLAAC address the prefix would produce.
- Use Split with a parent prefix and a child prefix length to list the first child subnets, then click Copy on any row or Copy all CIDRs.
- Open Reference for a quick read on /32, /48, /56, /60, /64, /127, /128, and the other prefixes that come up in real allocations.
About this tool
IPv6 Subnet Calculator handles the IPv6 sizing math a network engineer, sysadmin, devops engineer, or student actually needs at a desk. Paste an IPv6 address or prefix in any common form (compressed with ::, fully expanded eight groups, embedded IPv4 in the last 32 bits, optional zone identifier, optional /prefix, optional brackets) and the tool computes the network and last address in RFC 5952 compressed form and fully expanded form, the exact total address count using BigInt math, the count of /64 subnets inside the prefix when the prefix is /64 or shorter, the reverse DNS (ip6.arpa) stem of the network for PTR records, the eight-group hextet mask for tools that do not accept a prefix length, and a binary breakdown of the network address grouped by hextet. A classification panel labels the prefix as Global Unicast (2000::/3), Unique Local (fc00::/7, RFC 4193), Link-local (fe80::/10), Multicast (ff00::/8 with the standard scope decoding), Loopback (::1), Unspecified (::), IPv4-mapped (::ffff:0:0/96), 6to4 (2002::/16), Teredo (2001::/32), or Documentation (2001:db8::/32, RFC 3849), so the answer to is-this-a-public-IPv6 is obvious at a glance. When the prefix is /64 or shorter the page also derives a Modified EUI-64 interface identifier from a MAC address (RFC 4291 Appendix A), inserting ff:fe in the middle and flipping the universal/local bit, and shows the full SLAAC address that a stateless host would auto-configure (modern operating systems prefer privacy addresses per RFC 8981, which the page calls out). A Split mode takes a parent prefix and a child prefix length and lists the first N equal-sized child subnets with their network in CIDR form and last address, so dividing a /48 into 256 /56 customer prefixes or a /56 into 256 /64 LANs is a single paste. A Reference mode opens the practical prefix-length table that comes up in design discussions: /32 RIR LIR allocation, /48 end-site assignment (RFC 3177 and RFC 6177), /52 medium enterprise, /56 typical residential ISP delegation, /60 small residential ISP delegation, /64 single LAN required by SLAAC, /96 NAT64 (RFC 6052), /112 small management subnets, /126 and /127 point-to-point inter-router links (RFC 6164), and /128 single host routes. All parsing, BigInt arithmetic, formatting, and rendering happens locally in your browser, so the production IPv6 plans, allocations, and addresses you paste here never leave your device. Useful for sizing a new /48 end-site, splitting an ISP-delegated /56 into LANs, validating a Cisco or FRR route advertisement, writing a firewall ACL, building a NAT64 deployment, generating PTR records for ip6.arpa, debugging a SLAAC address, learning IPv6 subnetting for an exam, or sanity-checking the prefix length on a change ticket.
Free to use. Works in your browser. No signup, no login.
Related tools
You may also like
Subnet Calculator
IPv4 CIDR network, broadcast, host range, mask, and binary view.
Open tool
DeveloperIPv6 Expander and Compressor
Expand, compress, classify, and derive reverse DNS for any IPv6 address with optional zone and CIDR prefix.
Open tool
DeveloperCIDR Range Calculator
Batch CIDR to IP range, range to CIDR, and contains test for IPv4 and IPv6.
Open tool
DeveloperIP Address Validator
Validate IPv4 and IPv6 addresses, classify reserved ranges, and batch check a list of IPs.
Open tool
DeveloperRandom IP Address Generator
Bulk random IPv4 and IPv6 generator with scope filters, CIDR limits, and JSON, CSV, SQL output.
Open tool
DeveloperMAC Address Generator
Bulk random MAC address generator with notation, casing, OUI lock, and the I/G and U/L bit controls.
Open tool