Since Joomla in its default configuration uses /administrator as the 'back end' login for its site, it makes sense if you have a static IP to simply block everyone except that IP. You can of course, given you know the subnet's used by your ISP, allow access for all of those, but whichever you choose the setup is simple.

Firstly connect to your site using whatever tool you have to be able to change files. Usually FTP, if you're using GEN then we have the file manager in the web console.

You need to add a file to the /administrator folder called .htaccess and you need to add the following content to it.

<Limit GET POST>
 order deny,allow
 deny from all
 allow from 10.20.30.40
</Limit>

Here, replace 10.20.30.40 with your IP address, OR use your subnet e.g. 10.20.30.0/24

Once this file is uploaded, from that point onwards only access from that IP (or those IP's) will be allowed for /administrator.

You can repeat the "allow from" line for additional IP addresses if you have a few.

This same method can of course be used for WordPress by placing the file in the wp-admin folder.