Paste a compressed or expanded IPv6 address, set a prefix, and copy the network, first and last address, and ip6.arpa PTR. This IPv6 CIDR calculator is part of the networking tools on TechOpt.io. Need IPv4 instead? Use the IPv4 CIDR calculator or the dual-stack CIDR calculator.
IPv6 CIDR calculator prefix lengths that matter
Providers typically hand out a /32, /48, or /56, per RFC 4291. You then carve /64 networks for each LAN. A /64 is the length SLAAC expects; a /128 is a single host, often a loopback or anycast service address.
| Prefix | Role |
|---|---|
/32 | Common ISP or homelab allocation from an upstream |
/48 | Site allocation; plenty of /64s for VLANs |
/56 | Typical residential PD (256 × /64) |
/64 | One LAN, guest SSID, or container bridge |
/127 | Point-to-point link (two addresses) |
/128 | A single address |
Compressed vs expanded
RFC 5952 says to drop leading zeros in each 16-bit group and replace the longest run of zero groups with ::. 2001:0db8:0000:0000:0000:0000:0000:0001 compresses to 2001:db8::1. Documentation and some ACLs want the expanded form; humans and most configs want compressed. The calculator shows both.
GUA, ULA, and link-local
Global unicast (GUA) is 2000::/3 — the addresses your ISP routes. Unique local (ULA) is fc00::/7, usually fd00::/8, for lab-only numbering. Link-local fe80::/10 is required on every interface and is not forwarded. 2001:db8::/32 is reserved for documentation; use it in examples, not on a live LAN.
FAQ
What is an IPv6 /64?
A /64 is the standard LAN prefix. The first 64 bits identify the network; the last 64 bits identify a host. SLAAC, most home routers, and guest Wi-Fi all expect /64. Do not use a longer prefix on a LAN if you want SLAAC to work.
Why is there no IPv6 broadcast address?
IPv6 dropped broadcast. Neighbor discovery and multicast replace it. Every address in a prefix is in the range; there is no reserved all-ones address the way IPv4 uses .255 on a /24.
What is a ULA address?
Unique Local Addresses (fc00::/7, usually fd00::/8) are the IPv6 cousin of RFC1918. They are not routed on the public internet. Use them for lab networks, overlay fabrics, or sites that need local IPv6 without a provider prefix.
How do I compress an IPv6 address?
Drop leading zeros in each group, then replace the longest run of all-zero groups with :: (RFC 5952). 2001:0db8:0000:0000:0000:0000:0000:0001 becomes 2001:db8::1. This calculator shows both forms.
Does this IPv6 calculator upload my addresses?
No. It runs entirely in your browser. Addresses and prefixes never leave your machine.
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