How to prevent users from accessing using url

Hello Community

    I have a intranet (Sharepoint).  If you type Servername:portnumber
you can access the intranet.

    But sometimes I don't want anyone to access the intranet
for example when I am doing maintenance.
How can I make so that when I don't want that url bring up the intranet
I can stop users from accessing the intranet using that url?

    Thank you
    Shabeaut