This plugin restricts access to a blog on a multisite network so you have to be logged in to view pages. You can also configure a set of IP addresses or ranges to match client IP addresses if you want to allow access to some clients by IP. Its main purpose is to allow users to keep blogs from being indexed by search engines when they are under development, but to allow logged in users (or clients from within an allowed IP range, if this is configured) to access the site to review it.
The plugin should be Network activated on a Multisite WordPress installation.
A list of allowed IP addresses can be added to the Privacy Settings page on the Network Dashboard (under Settings). Full IP addresses or partial IP addresses are allowed here, and are added to a text box, one per line. If the setting is enabled on the blog, client IP addresses are pattern-matched against the stored list, and if one of the allowed IP addresses or IP address fragments matches the start of the client IP address, the site is accessible by the client (without login).
Each blog on the network can be made private using the Privacy Settings page in the Settings menu of the blog Dashboard. This page contains two checkboxes, one to activate the privacy setting for the blog to only allow logged-in users to view the site, and another to allow network users to view the site (if this setting is configured at the Network level). If IP address filtering is not used, the second checkbox is hidden.