Please note that AT Internet recommends using first-party cookies, their third-party counterparts being blocked on an increasing number of browsers.
The use of a first-party cookie limits your ability to recognize a user to a single domain, subdomain or folder on a site. The ability to read and write the cookie is restricted by the browser to the scope defined within the cookie. Thus, cross-domain analyses are not possible.
Third-party cookies are deposited on a domain separate from the website's. AT Internet deposits Unique visitor identification cookies on the .xiti.com or .ati-host.net domains depending on the collection server.
This means AT Internet's servers have the ability to read all cookies on those domains no matter which site the visitor is accessing, allowing for cross-domain analysis.
First-party cookies do not allow any information to be communicated to other parties. The exchange of information is solely between the client (browser) and the server providing the pages upon loading the URL. In this case, the site itself must set the cookies (AT Internet's tag manages this automatically).
When a visitor returns, the site gathers the value contained in the cookie and sends it in the hit to AT Internet's servers (&idclient= parameter).
For example, visitor ID 123 loads the following URL: www.atinternet.com. A cookie is set on their browser with the domain .atinternet.com. The next day, the same visitor reloads www.atinternet.com on their browser, the site gathers the value "123" from the cookie and sends it in the hit (&idclient=123). Once this information is processed the visitor will be recognized as visitor ID 123.
If you set cookies from several subdomains with the same site ID, you will get conflicting and duplicated IDs. Since the subdomains cannot share cookie information between them, a new cookie will be set for each. To avoid this effect, using www.atinternet-solutions.com and apps.atinternet-solutions.com as an example, make sure to use the domain rather than the subdomains with a period in front to include all possible subdomains when setting the cookie: ".atinternet-solutions.com"
For subdomains with differing site IDs, this will only have an effect for multi-site User Insights analyses based on Unique visitors. In that case, it would be best to use Identified visitor tracking, which is the most precise visitor tracking feature to date.