network_home_url( string $path = '', string $scheme = null )

function network_home_url( $path = '', $scheme = null ) {
	if ( ! is_multisite() )
		return home_url($path, $scheme);

	$current_network = get_network();
	$orig_scheme = $scheme;

	if ( ! in_array( $scheme, array( 'http', 'https', 'relative' ) ) )
		$scheme = is_ssl() && ! is_admin() ? 'https' : 'http';

	if ( 'relative' == $scheme )
		$url = $current_network->path;
	else
		$url = set_url_scheme( 'http://' . $current_network->domain . $current_network->path, $scheme );

	if ( $path && is_string( $path ) )
		$url .= ltrim( $path, '/' );

	/**
	 * Filters the network home URL.
	 *
	 * @since WP-3.0.0
	 *
	 * @param string      $url         The complete network home URL including scheme and path.
	 * @param string      $path        Path relative to the network home URL. Blank string
	 *                                 if no path is specified.
	 * @param string|null $orig_scheme Scheme to give the URL context. Accepts 'http', 'https',
	 *                                 'relative' or null.
	 */
	return apply_filters( 'network_home_url', $url, $path, $orig_scheme);
}