Source

File: wp-includes/functions.php

function is_main_network( $network_id = null ) {
	if ( ! is_multisite() ) {
		return true;
	}

	if ( null === $network_id ) {
		$network_id = get_current_network_id();
	}

	$network_id = (int) $network_id;

	return ( get_main_network_id() === $network_id );
}