Source

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

public static function get_instance( $network_id ) {
		global $wpdb;

		$network_id = (int) $network_id;
		if ( ! $network_id ) {
			return false;
		}

		$_network = wp_cache_get( $network_id, 'networks' );

		if ( false === $_network ) {
			$_network = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM {$wpdb->site} WHERE id = %d LIMIT 1", $network_id ) );

			if ( empty( $_network ) || is_wp_error( $_network ) ) {
				$_network = -1;
			}

			wp_cache_add( $network_id, $_network, 'networks' );
		}

		if ( is_numeric( $_network ) ) {
			return false;
		}

		return new WP_Network( $_network );
	}