I was given a delegated subnet by AT&T and needed to subnet it for the Fortinet 60E 8 layer 3 LAN ports and the 221-C Wireless access point.  I will explain the process of subnetting a /64 block into /67.

IPv6
Status Available
Global Unicast IPv6 Address 2600:1702:980:25e0::1
Link-local IPv6 Address fe80::eea:c9ff:feb8:84e0
IPv6 Addressing Subnet (including length) 2600:1702:980:25e0::/64
IPv6 Delegated Prefix Subnet (including length) 2600:1702:980:25ef::/64
IPv6 Delegated Prefix: 
2600:1702:980:25ef/64
Start Range:  2600:1702:980:25ef:0:0:0:0
End Range:  2600:1702:980:25ef:ffff:ffff:ffff:ffff
No. of host:  18446744073709551616

I had it translated into words
eighteen quintillion four hundred forty-six quadrillion seven hundred forty-four trillion seventy-three billion seven hundred nine million five hundred fifty-one thousand six hundred sixteen
Subnet this into 8 networks
2^3 = 8
You take the power of 3 and add it to 64
/64 + 3 = /67

You take the block to the left of 25ef
You create an interesting mask 1110b nibble

8 4 2 1 16 constant
1 1 1 0 = 14 mask nibble

subtract them
16 constant
- 14 mask nibble
2 interesting nibble

The red numbers are what get changed with the interesting nibbles.

2600 1702 980 25ef 0000 /67 
2600 1702 980 25ef 2000 /67 
2600 1702 980 25ef 4000 /67 
2600 1702 980 25ef 6000 /67 
2600 1702 980 25ef 8000 /67 
2600 1702 980 25ef A000 /67 
2600 1702 980 25ef C000 /67 
2600 1702 980 25ef E000 /67
First Subnet
2600 1702 980 25ef 0000 0000 0000 0000 to 2600 1702 980 25ef 1FFF FFFF FFFF FFFF

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