It's crucial to use alt tag and to make sure search engines know what you are talking about. Alexa rank is a scoring framework that measuring an activity and making characterization as per this movement those Alexa Toolbar clients are going to sites.
Domain is either you or your company's ID in the Internet, when you apply to connect to Internet, they will give you protocol number. Every device connected to Internet has an IP number. Webmasters, publicists and ad networks assess your website that base on alexa rank estimation of your site or online journal.
If there is no alt tag, search engine doesn't understand anything from the image.
That's why you should use alt tag on your all pictures, which you use in your website.
The links on the other websites are called external link.