Since it seems that you acknowledged what I last typed on the last part, I believe you would need more information.
Now, how this would work is by using .htaccess, the IP allow/deny "commands," and an custom error document. Plenty of guides online of how to pull it off.
You will need to create a file named '.htaccess' (exactly as put in the single quotes) and enter something like this in the file...
Code:
order deny,allow
deny from all
allow from 255.255.255.255
ErrorDocument 403 /maintenance.html
Replace 255.255.255.255 with your PUBLIC IP address. You can get this by googling 'ip'.
Also, 'maintenance.html' is an example file that people would end up on (who aren't using the whitelisted IP). You can place this file in a folder or/and rename it, though you would need to change the path set in the .htaccess file.
If you already have a page created for users who aren't you to end up landing in, change the path so that it's pointing to that.