Wordpress: difference between site_id and blog_id?

I'm trying to get some understanding of how Wordpress works and while inspecting the database I found that, in a multisite ins开发者_如何学运维tallation, there is a table called wp_site that seems always holding a single record with a corresponding site_id. I cannot understand (nor find on internet) what is the use of site_id when actually multisites seem to use wp_blogs and corresponding blog_id for identifying different sites.(http://codex.wordpress.org/Database_Description)

Can someone shed some light on what should be my understanding of site_id?

Thanks

Giuseppe

site is a reference to a network of blogs. There are efforts under way to allow WordPress to support multiple networks in addition to multiple blogs, but right now that's not a stock feature (which is why the wp_site table only contains one value.

The wp_blogs table contains all of the blogs for your entire installation, with each identified by the site it's associated with.

This gives you a clean separation between networks, and (eventually) you'll be able to have different "groups" of blogs on one WordPress installation behave like distinct multisite installations.