Octet

Say this first

128 bits written as eight groups of 16 bits (hextets), each as up to four hex digits, separated by colons. The prefix length follows a slash, like 2001:db8:acad:10::1/64. On a LAN the first 64 bits are the network prefix (global routing prefix plus subnet ID) and the last 64 bits are the interface ID.

Go deeper

The pieces

Output

2001:0db8:acad:0010:0000:0000:0000:0001/64
|-- global routing prefix --|subnet|----- interface ID -----|
     48 bits                 16 bits        64 bits

Each hex digit is 4 bits (a nibble), so prefix lengths on multiples of 4 line up with digits. That is why address plans use /48, /52, /56, /60, /64.
Letters are case-insensitive. Write them lowercase (RFC 5952). Cisco shows them uppercase anyway.
There is no subnet mask. You always write the prefix length.

Where the notation bites

URLs: wrap the address in brackets so the colons do not look like a port: http://[2001:db8::10]:8080/.
Link-local: the same fe80:: address can exist on every interface, so hosts need a zone ID: ping fe80::1%eth0 on Linux, ping fe80::1%12 on Windows.
IPv4 inside IPv6: the last 32 bits may be written dotted, as in ::ffff:192.0.2.1 or 64:ff9b::192.0.2.33.

Quick numbers

Block	Contains
/32	65,536 /48s
/48	65,536 /64s
/56	256 /64s
/64	2^64 addresses, about 1.8 x 10^19

They will ask next

How many /64 subnets does a site with a /48 have?
2^(64-48) = 2^16 = 65,536.
Why is the interface ID 64 bits?
SLAAC and EUI-64 were designed around a 64-bit interface ID (RFC 4291 requires it for most unicast space). Using a different LAN prefix length breaks SLAAC and surprises a lot of software.

Traps

Calling the groups octets. They are 16 bits each.
Writing an address in a URL without brackets.

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