If you already have a website and want to keep it where it is, but you also want to use your custom domain name with email on Office 365 operated by 21Vianet, Internet traffic has to be redirected to your website from Office 365. (To learn why, check out this topic: Why do I need to complete these steps?)
Because of the way that domains and DNS work, we have to use a DNS record to redirect people to your website. And to create the DNS record, we need to know either the IP address of your website or the fully qualified domain name. We’ll use this information to point to your site from Office 365.
Which one we need depends on the company that currently hosts your website. Sometimes website hosting providers have a single static IP address that is unique for each website they host; for example, 192.168.100.1. In this case, you’ll need the IP address for your website.
Caution: If your web hosting provider does provide an IP address for your website, make sure that it’s a static IP. That is, it must be an IP address that always points to your website rather than being available to be assigned to different website addresses. The web hosting company can confirm this for you.
Sometimes instead of using static IP addresses, companies use a different way of identifying each website: a fully qualified domain name. It’s different from your custom domain name. A fully qualified domain name is simply an address name where some companies host websites. It looks something like this: sites.<web hosting company>.com.
If you’re given a fully qualified domain name for your website instead of an IP address and we use the fully qualified domain name to redirect people to your site, be aware that if people type your domain name without including “www.”, they won’t get to your site. For example, http://fourthcoffee.com won’t be redirected to http://www.fourthcoffee.com.
How can you fix this? See Make sure people can get to your website outside Office 365 when the address includes www.