Why use this IPv6 subnet calculator?
IPv6 addresses are 128 bits long and written in hexadecimal. Calculating subnets by hand is error-prone and slow. This calculator does it in one click: enter a CIDR block, get every detail instantly.
Use cases:
- Network planning: size /48, /56 or /64 subnets before deploying infrastructure
- Reverse DNS: generate the exact ip6.arpa entry for PTR record configuration
- Address verification: confirm an IPv6 prefix is valid before pushing it to production
- Documentation: export expanded, compressed and full forms for addressing plans
What is an IPv6 address?
An IPv6 address is a 128-bit identifier defined by RFC 4291. It is written as 8 groups of 4 hexadecimal digits separated by colons. The :: shorthand compresses consecutive groups of zeros into a single token.
Example:
Compressed: 2001:db8::1
Expanded: 2001:0db8:0000:0000:0000:0000:0000:0001
The CIDR prefix (e.g. /64) sets the boundary between network bits and host bits. A /64 prefix means 64 bits for the network, 64 bits for hosts.
How to calculate an IPv6 subnet
Every IPv6 subnet calculation starts with the CIDR prefix length. The formula is straightforward:
- Identify the prefix: the first N bits define the network portion
- Calculate host bits: 128 minus N equals the bits available for hosts
- Determine the range: network address (all host bits = 0) to last address (all host bits = 1)
Example: 2001:db8:abcd::/48
| Element | Value |
|---|---|
| Prefix | /48 (48 network bits) |
| Host bits | 80 bits |
| Total addresses | 2^80 (1,208,925,819,614,629,174,706,176) |
| Possible /64 subnets | 65,536 |
Enter 2001:db8:abcd::/48 in the calculator above to verify these values.
Common IPv6 prefix lengths at a glance
IANA and Regional Internet Registries (RIRs like RIPE, ARIN, APNIC) assign IPv6 blocks following these standard sizes:
| Prefix | Typical use | Addresses | /64 subnets |
|---|---|---|---|
| /128 | Single host (loopback, host route) | 1 | 0 |
| /127 | Point-to-point link (RFC 6164) | 2 | 0 |
| /64 | Standard LAN subnet (RFC 4291) | 2^64 | 1 |
| /56 | Residential customer allocation | 2^72 | 256 |
| /48 | Site allocation | 2^80 | 65,536 |
| /32 | ISP allocation from RIR | 2^96 | 4.3 billion |
The /64 boundary is not arbitrary. SLAAC (Stateless Address Autoconfiguration, RFC 4862) requires exactly 64 host bits to function.
Key differences between IPv4 and IPv6 subnets
IPv6 eliminates several IPv4 constraints. There is no broadcast address, no subnet mask in dotted-decimal, and no need to subtract 2 from the address count.
| Feature | IPv4 | IPv6 |
|---|---|---|
| Address size | 32 bits (4.3 billion addresses) | 128 bits (3.4 x 10^38 addresses) |
| Mask notation | Dotted decimal or CIDR | CIDR only (/N) |
| Broadcast | Yes (last address reserved) | No (replaced by multicast, RFC 4291) |
| Standard subnet | /24 (254 usable hosts) | /64 (2^64 hosts) |
| Usable addresses | Total minus 2 (network + broadcast) | All addresses in the block |
| Auto-configuration | DHCP only | SLAAC (RFC 4862) or DHCPv6 |
IPv6 address types
This calculator detects the address type automatically based on the prefix, as defined in RFC 4291 and RFC 4193:
| Type | Prefix | Use |
|---|---|---|
| Global Unicast | 2000::/3 | Publicly routable addresses (internet traffic) |
| Link-Local | fe80::/10 | Communication within a single network segment |
| Unique Local (ULA) | fc00::/7 | Private addresses, not routed on the internet (RFC 4193) |
| Multicast | ff00::/8 | One-to-many delivery to a group of nodes |
| Loopback | ::1/128 | Local interface only (equivalent of 127.0.0.1) |
| IPv4-mapped | ::ffff:0:0/96 | IPv4 address embedded in IPv6 format |
If the calculator flags an address as "deprecated" or "reserved," check the IANA IPv6 Special-Purpose Address Registry for current status.
FAQ: frequently asked questions
Q: What is an IPv6 subnet?
A: A logical division of the 128-bit IPv6 address space. The standard subnet size is /64, providing 2^64 addresses per LAN as specified in RFC 4291.
Q: What prefix length should I choose?
A: Use /64 for LANs (required by SLAAC), /48 for a site, /56 for a residential customer, /32 for an ISP allocation, /128 for a single host route.
Q: How does reverse DNS work for IPv6?
A: Each nibble (4 bits) of the address is reversed and separated by dots under the ip6.arpa domain. This calculator generates the full PTR entry automatically.
Q: Does IPv6 use subnet masks?
A: No. IPv6 uses CIDR prefix notation (/N) exclusively. Dotted-decimal masks and wildcard masks do not exist in IPv6.
Complementary tools
| Tool | Purpose |
|---|---|
| IPv4 netmask | Calculate network, broadcast and wildcard for an IPv4 address |
| Reverse DNS | Find the hostname associated with an IP address |
| My IP address | Display your public IP address (IPv4 and IPv6) |