By default, Matomo (Piwik) will match a Unique Visitor based on the Visitor ID cookie (if visitor supports cookies) and/or based on a user fingerprint processed from the IP address and user device information (if visitor does not support cookies).
There are a few use cases when the default Tracker algorithm to track unique visitors does not work as expected. For example if you use Matomo on an Intranet and all users have the same IP address, or if many of your visitors come from a Company or University network and have all the same IP addresses, then Matomo may track many users as one unique visitor.
In these cases you can enable better unique visitors recognition:
If your website has a login form, when users sign-in we recommend to tag your users with a User ID.
Alternatively if you cannot use User ID then you may force Matomo not use the IP address and device info to decide Unique Visitors but instead rely on the Visitor ID found in the user cookie. To force Matomo to not rely on the IP address as a mean to differentiate one user from another, edit your config/config.ini.php and add a new section
[Tracker]with the following:
[Tracker] trust_visitors_cookies = 1