Multiple Instances on Subdomains sharing domain

It’s been a while that I’m asking myself if it would be possible to do:

Each one of them having their own login website, but all of them sharing same

For example, for schools:

Would they need to share database? Would be that possible with LDAP?



You can use LDAP to log in, but you cannot share the identifier. If the message should go to, which instance should receive it?