Good evening @lab.8916100448256,
Thank you very much for the reply and the tip 
If I learn, I know how to reuse and adapt, but I’m a Linux novice at the moment 
The day before yesterday, I learned about the ifconfig command, but I couldn’t use it. Yesterday, I learned that I now have to use ip. That’s how I was able to provide the information this morning.
The Yunohost documentation¹ says to modify the /etc/network/interfaces file.
But when I browse the folder, I only see three subfolders: if-down.d / if-pre-up.d / if-up.d
In my Infomaniak interface, in the Network section, I find: ext-net1-v6subnet1 2001:1600:16:10::/64 IPv6 2001:1600:16:10::1
The IPv6 address mentioned is shorter than the public address to join my Yunohost server: 2001:1600:16:10::603/128
If I’m not mistaken, I should add:
iface enp0s3 inet6 static
address 2001:1600:16:10::603
netmask /128
post-up /sbin/ip -6 route add 2001:1600:16:10::1 dev enp0s3
post-up /sbin/ip -6 route add default via 2001:1600:16:10::1 dev enp0s3
pre-down /sbin/ip -6 route del default via 2001:1600:16:10::1 dev enp0s3
pre-down /sbin/ip -6 route del 2001:1600:16:10::1 dev enp0s3
Is that right? But now I don’t know where the file to modify is 
Thanks in advance for the help and sorry for the beginner that I am 
¹ I found a bad link on the French documentation page: Configuration de l'IPv6 | Yunohost Documentation
Configure the DNS server
here: YunoHost: garden your own piece of the Internet!
should instead redirect here in my opinion: Configuration de la zone DNS | Yunohost Documentation
I tried to do it myself so as not to disturb, I connected to GitHub, but after that I don’t know whether to Fork this repository and edit the file or Edit in place and I especially don’t want to break anything in the community 