SubnetKit › IPv6 /64 Subnet Calculator
IPv6 /64 Subnet Calculator: how it works
The /64 is the standard IPv6 subnet size for any LAN, wireless segment or VM network. It is fixed at 64 bits because Stateless Address Autoconfiguration (SLAAC) builds the host portion from a 64-bit interface identifier; anything longer than /64 breaks it.
SubnetKit works with the full 128-bit address space in your browser, free of charge, and the IPv6 tab above is loaded with the documentation prefix 2001:db8:abcd:12::/64.
Worked example: 2001:db8:abcd:12::/64
- Network: 2001:db8:abcd:12::/64
- Expanded form: 2001:0db8:abcd:0012:0000:0000:0000:0000
- First address: 2001:db8:abcd:12::
- Last address: 2001:db8:abcd:12:ffff:ffff:ffff:ffff
- Addresses: 2^64 = 18,446,744,073,709,551,616
- Type: global unicast (2001:db8::/32 is reserved for documentation)
The first four hextets (64 bits) are the routing prefix and subnet ID; the last four are the interface identifier. Routers advertise this prefix on the link and hosts generate their own addresses, so you never run out of host addresses in a /64 and there is no concept of a broadcast address.
How many /64 subnets you get
Allocations are usually expressed by how many /64s they contain. Each 4 bits of prefix equals one hex digit, so the arithmetic is quick:
- /48 (typical business site): 2^16 = 65,536 /64 subnets
- /56 (common residential allocation): 2^8 = 256 /64 subnets
- /60 (some ISPs): 2^4 = 16 /64 subnets
- /32 (ISP allocation from a RIR): 2^32 = 4,294,967,296 /64 subnets
In SubnetKit, enter a /48 or /56 and set the split prefix to 64 to list the first subnets of the range with their boundaries. Use a /127 for point-to-point router links (RFC 6164) and a /128 for loopbacks; everything with end hosts stays /64.
Frequently asked questions
How many addresses are in an IPv6 /64?
2^64, which is 18,446,744,073,709,551,616. There is no broadcast and no reserved network address, though the all-zeros interface identifier is conventionally avoided.
Why must a LAN be a /64?
SLAAC derives a 64-bit interface identifier from the MAC address or a random value, so autoconfiguration only works when exactly 64 bits remain for the host portion.
How many /64s are in a /48?
65,536. The 16 bits between /48 and /64 form the subnet ID, usually written as the fourth hextet.
Can I use /127 or /126 on router links?
Yes, RFC 6164 recommends /127 for point-to-point links between routers to avoid neighbor-cache exhaustion attacks. Host segments should still be /64.
Looking for the full guide, tips and the complete FAQ? Read the main SubnetKit page.