Why is a /64 the usual LAN size?

Stateless address autoconfiguration expects a 64-bit interface identifier, so a /64 leaves room for it. Remember: /64 for a LAN, /48 or /56 for a site.

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IPv6 / CIDR calculator

Paste an IPv6 address and a prefix. See the compressed form, the full address, the network boundary and the last address in the range. No signup and no server round trip.

Calculate a range

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IPv6 addressPrefix

2001:db8:acad:12::/64

Compressed: 2001:db8:acad:12::1
Expanded: 2001:0db8:acad:0012:0000:0000:0000:0001
Network: 2001:db8:acad:12::
Last address: 2001:db8:acad:12:ffff:ffff:ffff:ffff
Prefix: /64
Range size: 18,446,744,073,709,551,616 addresses

The range size is 2 to the power of the host bits. A /64 is 18,446,744,073,709,551,616 addresses. The maths is exact; the display rounds nothing.

Reading the result

The network is the address with every host bit set to zero. The last address is the network with every host bit set to one. In IPv6 there is no broadcast address, so both ends of the range can be assigned unless a protocol reserves them.

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Read the IPv6 lessonNetwork+ path: addressing and prefixes Try an addressing labConfigure and verify a small range

Questions

The prefix counts the leading network bits out of 128. A /64 leaves 64 host bits, which is the standard size for one LAN. A /128 names a single address. The calculator masks the address and shows the first and last address in the range.

Addressing is the hinge of the Network+ and CCNA paths

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