🌐 IPv6 Subnet Calculator
Expand an IPv6 address, parse a CIDR block, and see the first and last address, network range, host count, and reverse DNS.
Last updated: May 21, 2026 · By Λ
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IPv6 addresses are 128 bits, written as eight groups of four hex digits separated by colons, with leading zeros omitted and the longest run of zero groups replaced by a double colon. The compact notation is easy to read but hard to reason about when you need to know what falls inside a /64 or a /48. This calculator parses an address in any valid form and walks the prefix length to show the network address, the first and last addresses in the block, the address count, the prefix mask, and the reverse-DNS pointer name for the network. The math is exact (uses 128-bit BigInt), so you can paste any prefix from /4 up to /128 without rounding errors.
Most homes get a /64 from their ISP. Many ISPs give a /56 or /48 if you ask, which gives you 256 or 65536 /64 subnets respectively. Enterprise networks routinely allocate /48s per site. Knowing where your prefix sits in the larger hierarchy matters when you are setting up router advertisement scopes, configuring firewall rules, or troubleshooting why a SLAAC client picked an unexpected address. The reverse DNS field gives you the in-addr-style PTR record that you would publish for the network.
How to use this tool
- Type or paste an IPv6 address into the box. You can include a CIDR prefix, like 2001:db8::1/64. If you leave the prefix off, /128 is assumed.
- Results refresh as you type. Errors show in red if the address is malformed.
- Use the sample buttons to load common test prefixes.
- Click any value to select it for copying.
Frequently Asked Questions
What is a /64 and why is it everywhere?
A /64 is a 64-bit prefix, leaving 64 host bits, which equals about 18 quintillion addresses. The IPv6 architecture defines /64 as the standard size for a single network segment because mechanisms like SLAAC (stateless address autoconfiguration) depend on having 64 host bits. Routers between segments typically allocate /64s out of a larger /56 or /48.
Why does IPv6 have so many addresses?
2 to the 128th power is roughly 340 undecillion. The design intent was to never run out of addresses again, and to allow generous sparse allocation so providers can hand out /48s and /56s without thinking about scarcity. The downside is that compact notation hides the structure, which is why a calculator like this is useful.
What does "expanded" mean?
The expanded form writes every group as four hex digits with no double colon abbreviation. 2001:db8::1 expanded is 2001:0db8:0000:0000:0000:0000:0000:0001. This form is unambiguous and is what you would use as a sort key or in a record that does not understand the compact form.
What is reverse DNS for IPv6?
IPv6 reverse DNS uses the ip6.arpa zone with each nibble (4 bits) written as a single hex digit in reverse order. The /64 network 2001:db8::/64 becomes 0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. The calculator computes this pointer for you so you can paste it into a DNS zone.