Also you need to create a DNS record to your (www) subdomain.

For intance: example.com is your domain (zone) and www.example.com is your subdomain such as api.example.com or w3.example.com, but the commonly used subdomain is www by convention.

Let’s say you have a A record in your DNS for your domain like this

@    A    192.168.0.1

People can reach your site just typing http://example.com in the URL bar of the browser. Now if you need to expose your site as http://www.example.com you can do this in several ways, these are two of them.

1. By adding a new A record in your DNS zone.

   www    A    192.168.0.1
   

2. By adding a CNAME record in your DNS zone.

   www    CNAME @
   

I would prefer the option 2 but it’s your choice.

Hope this helps you. :)

Add ServerAlias www.example.com to your virtual host file to listen on www.

For example:

<VirtualHost *:80>
    ServerAdmin admin@example.com
    ServerName example.com
    ServerAlias www.example.com
    DocumentRoot /var/www/example.com/public_html
    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>