[TL;DR - it’s best not to touch it in your Fritz!Box settings]
An IPv6 interface ID is simply the part of the IPv6 address that comes after the subnet prefix. Let’s call it the suffix.
So, if the assigned prefix is 2001:db8:1111:2222::/64 and the suffix is aaaa:bbbb:cccc:dddd, that would lead to the complete IPv6 address 2001:db8:1111:2222:aaaa:bbbb:cccc:dddd/64.
IPv6 interface IDs basically come in three flavours:
- Configured manually (including DHCPv6)
- Modified EUI-64 (in other words: based on the MAC address)
- (Semi) randomly picked automatically
Manual configuration allows you do do cool stuff like:
2001:db8:1111:2222:0:0:0:123/64 (or 2001:db8:1111:2222::123/64)
Modified EUI-64 based suffixes are used for automatically generating a unique IPv6 address based on your MAC address (this is called SLAAC). This is often the default. It’s not exactly your MAC address that shows up in the generated IPv6 address, there’s some processing done, but the MAC address can easily be derived from it.
Because your MAC address can be derived from the SLAAC generated IPv6 address, you could in theory be tracked when you move around networks. For mobile devices this might be undesired. That’s why they invented ‘privacy extensions’ (RFC4941), which picks random values (that also change at certain intervals). Because of this, you can no longer be tracked based on your MAC address. But it goes without saying that RFC4941 addresses are not suitable for offering services, such as a public NTP server.
Regular changing addresses also impose other challenges. For example in the case of an IP whitelist. It’s impossible to whitelist a client when it’s IPv6 address constantly changes to something random. So, there is also a semi random way (RFC7217) that makes sure your IPv6 address can’t be tracked over various networks, while it remains constant in every individual network. So for example at home you always get the same IPv6 address (assuming your prefix hasn’t been changed by your ISP) and at work you also get the same address. Both the prefix and suffix are different and the MAC address is not used for it. Hence, it is impossible to track you and know that you where traveling between home and work.
Fritz!Box CPE’s have an IPv6-interface-id field, that by default is filled with the EUI-64 derived value. But it can be changed. I’d recommend not touching it, especially when you have port sharing enabled, to avoid mismatches and confusion.