IPv6 Subnet Calculator
Expand or compress any IPv6 address, inspect its prefix and type, count addresses and /64 subnets — instantly and server-side.
Results for 2001:db8::/56
- Fully expanded form
- 2001:0db8:0000:0000:0000:0000:0000:0000
- Compressed form (RFC 5952)
- 2001:db8::
- Prefix length
- /56
- Type
- Documentation (RFC 3849)
- Total addresses
- 4,722,366,482,869,645,213,696
- /64 subnets inside
- 256
- First address of prefix
- 2001:db8::
- Last address of prefix
- 2001:db8:0:ff:ffff:ffff:ffff:ffff
- Network in CIDR notation
- 2001:db8::/56
Keep exploring IPv6
Reference pages for the most common allocations: IPv6 /32 IPv6 /48 IPv6 /56 IPv6 /64
Working with IPv6 prefixes
An IPv6 address is 128 bits long and written as eight hexadecimal groups. Any single run of all-zero groups can be squeezed into ::, which is why 2001:0db8:0000:0000:0000:0000:0000:0001 and 2001:db8::1 are the same address. A prefix written with a slash (“/48”) fixes that many leading bits, leaving the rest for subnets and hosts.
The /64 boundary is special. Interface identifiers are 64 bits wide, so a /64 is the standard subnet size — the smallest prefix that supports SLAAC, the mechanism that lets hosts configure their own addresses from router advertisements. Networks are therefore planned hierarchically: an ISP holds a /32, hands each customer site a /48 or /56, and every LAN inside it becomes one /64 containing 18 quintillion addresses.
Special ranges to know: 2001:db8::/32 is reserved for documentation (RFC 3849), fe80::/10 is link-local, fc00::/7 holds unique-local addresses, and ::1 is loopback — this tool labels them automatically.