Apache VirtualHost : multiple sites on same IP - apache

Question

Let assume that blah.com, blah2.com all point to the same server with IP=5.31.111.7.
I would like that:
accessing blah.com serves /var/www/site1
accessing blah2.com serves /var/www/site1
accessing 5.31.111.7 serves /var/www/site2
I tried
<VirtualHost *:80>
DocumentRoot /var/www/site1
</VirtualHost>
<VirtualHost 5.31.111.7:80>
DocumentRoot /var/www/site2
</VirtualHost>
but now everything goes to /var/www/site2, which is not what I wanted.
How to configure the VirtualHost, such that the served website depends on the URL ?
PS: why should I do this in /etc/apache2/sites-enabled/000-default instead of /etc/apache2/apache2.conf ? I don't understand this sites-enabled / sites-available/default naming... Why are there so many different config files by default on Debian, for such a simple thing?

Answer 1

What you want to do is called Name-Based Virtual Hosting, you'll need
NameVirtualHost *:80
to enable it on port 80, and for each VirtualHost, you need to give the name(s):
<VirtualHost *:80>
ServerName blah2.com
ServerAlias www.blah2.com
DocumentRoot /var/www/site1
</VirtualHost>
Note that there are limitations on SSL/TLS when doing name-based virtual hosting, but it's a bit of a moot point since post-POODLE, people start to require TLS anyway, so ancient browsers are out of luck anyway.
As to the config files, it's very very useful to have two classes of config files: the ones with defaults that a package update will overwrite, and your local ones that it will not touch, or better even, a directory full of the former and a directory full of the latter. (Because additional packages might want to make configuration settings, they'll all install in the former place, and you should only ever change/override config in the second place.)