Multisite

It is enabled with WP_ALLOW_MULTISITE and then the Network Setup screen, which adds MULTISITE, SUBDOMAIN_INSTALL, and DOMAIN_CURRENT_SITE constants to wp-config.php. Tables for site 2 are named wp_2_posts, wp_2_options, and so on, while wp_users, wp_usermeta, wp_blogs, and wp_site are shared. Uploads go to wp-content/uploads/sites/2.

List the sites with wp site list or look at Network Admin > Sites. The count of rows in wp_blogs is your site total.

A university runs a network of 240 department sites at subdomains like physics.example.edu. A security update to one plugin is applied once and reaches all 240. The same plugin on a single shared host would have been 240 separate updates.

The cost shows up in the database: the options table for each site autoloads separately, and a runaway cron on site 87 slows every site because they share one MySQL server.

Multisite is one codebase with many sites, while a plain WordPress install is one site with one set of tables. Multisite shares plugins and users but not posts, so a post on site 3 is not on site 4. Moving a single site into or out of a network needs an export and a table rename.

Choosing subdomains when the DNS does not support wildcard records. Activating plugins network-wide that only one site needs. Assuming a plugin works on a network because it works alone. Forgetting that a Super Admin can reach every site. Trying to back up a single site by copying a folder.

Before you convert, decide whether the sites truly share an owner and a plugin set. If they have different clients with different needs, separate installs avoid shared failure. If you proceed, test with a network of two sites and check the plugins you rely on.