Occasionally websites will redirect you to a version of their URL that replace the regular old WWW with a WWW1, or WWW2, or rarely even 3, 4, 5 and onwards. In a world with increasingly elaborate online scams involving fake emails and domains, these unexpected numbers can feel like cause for concern. So what's the actual deal with WWWX domains?
You may have noticed these kinds of domains popping up especially in online banking applications (Citibank's homepage will redirect straight to a WWW1, for example), which are the type of sites you really want to make sure are secure. The good news is that these URLs don't mean the page is unsafe or compromised.
WWW1 or WWW2 and the like are hostnames or subdomains, used for load balancing on domains with large user loads. The number indicates that the data your web browser is accessing is being sent from a different webserver than the one serving the main WWW domain. These kinds of subdomains are often used for secure webpages, which is why you'll see it more often on internet banking than anywhere else.
Of course there are other, more popular methods of load balancing large websites these days, which is why you won't see these kinds of subdomains on sites like Facebook, for example. When you do see it pop up on your bank's log in page, however, there's no need to panic - everything's as it should be.