By default, Piwik will use the IP address to decide Unique Visitors. There are a few use cases where using the IP address to decide unique visitors can be problematic. For example if you use Piwik 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.
There are some solutions to enable better unique visitors tracking in those cases:
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 Piwik not to use the IP address to decide Unique Visitors but instead rely on the Visitor ID found in the user cookie. To force Piwik 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:
trust_visitors_cookies = 1