A gut reaction may be, “what is this, 1999?”
But for those of us who have been webmasters since that time, this is not a given. And I recently came across an interesting use case that made me reverse my assumptions.
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Generally I would say, “No! Don’t use www. in your domain name.” Simpler is better. Nobody says “double-yew-double-yew-double-yew-dot-facebook-dot-com” – they just say “Facebook” and you likely open the app – or google / search for “Facebook” and click the first link.
Same with email – no one emails anyone@www.domain.com.
But is it really that simple? Here are some other considerations.
If your target audience is slightly older — say, 50-somethings — then they may type www anyway, even if your website doesn’t exist on that subdomain! (Make sure you’re doing proper port bindings / redirects!)
Also from a marketing point of view, especially if you’re using a novel TLD – like .io – it may not be immediately apparent to some that it’s a website. (Like codepen.io).
But recently I came across an interesting use case. We were updating SSL certificates for a web application, and we had to renew and implement two separate certificates — one wildcard cert, for the umpteen subdomains in use — as well as a ‘canonical’ one, for just the plain site with no www or subdomain.
If you’re not using Let’s Encrypt / ACME certs and paying a CA, you may be paying twice as much! Why not just serve everything from from www., which would be covered under the wildcard cert?
Originally, the www was meant to be your HTTP port (80/443), versus other services on your server (such as LDAP / AD domain or FTP), which may have shared that same IP address.
Conclusion: If you’re ONLY running a standalone website, there is likely no need to run www. But if you’re running subdomains, you may want to consider using www for your main site, to prevent the need for a standalone certificate.