The AT&T router has IPv6.   Fortinet and I setup WAN and LAN side IPv6.

This is what was on the AT&T modem.

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
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

Here is the first subnet range

2600 1702 980 25ef 0000 0000 0000 0000 to 2600 1702 980 25ef 1FFF FFFF FFFF FFFF

Staying with my naming convention of old.

port, ipv4, ipv6
1 172.16.5.0/24 2600:1702:980:25ef:0172:16:5:0/67
2 172.16.1.0/24 2600:1702:980:25ef:2172:16:1:0/67
3 172.16.2.0/24 2600:1702:980:25ef:4172:16:2:0/67
4 172.16.3.0/24 2600:1702:980:25ef:6172:16:3:0/67
5 172.16.10.0/24 2600:1702:980:25ef:8172:16:10:0/67
6 192.168.0.0/24 2600:1702:980:25ef:a192:168:0:0/67
7 192.168.3.0/24 2600:1702:980:25ef:c192:168:3:0/67
DMZ 192.168.1.0/24 2600:1702:980:25ef:e192:168:1:0/67