Source

File: wp-includes/class-wp-network.php

public function __get( $key ) {
		switch ( $key ) {
			case 'id':
				return (int) $this->id;
			case 'blog_id':
				return (string) $this->get_main_site_id();
			case 'site_id':
				return $this->get_main_site_id();
		}

		return null;
	}