I was wondering if it's possible to create a user (or modify an existing one) in Ms Sql server that can only connect to the DDBB using an internal IP and not an external IP. Something similar to Mysql, that allows you to specify the host. Sorry if it is a very easy question but I am not expert at all in SQL server!
You'll need to clarify what you mean by "an internal IP".
If you mean, on the MS SQL box itself, this is trivial by only binding to localhost (127.0.0.1).
If you mean, from a list of allowed remote IPs - employ a firewall.
What on earth has possessed you to EVER open up MS SQL to an externally-reachable port.
I would do that via Windows firewall. It should have a default rule for SQL Server (assuming you didn't turn the FW off. You can limit it to IP ranges, etc.