snippet for functions.php or plugin to restrict user access

alternative.php

function restrict_access_if_logged_out(){
    if (!is_user_logged_in() ){
        $redirect = home_url() . '/wp-login.php?redirect_to=' . get_site_url;
        wp_redirect( $redirect );
     exit;
    } else {
        if (!current_user_can('manage_options') && !is_admin()) {
            show_admin_bar(false);
        }
}
add_action( 'wp', 'restrict_access_if_logged_out', 3 );

desc.md

Example of how to restrict access to a network or logged in user only

The code could be put into functions.php

Note this is an example, and untested in live and would need to be edited to the right network values

It also isn't that secure as the REMOTE_ADDR can be spoofed so shouldn't be used for really secure content - in which case alternative security measures ( i.e. Single Signon within an organisation) should be used

functions.php

function restrict_access_if_logged_out(){
    if (!is_user_logged_in() ){
        $ip = explode('.',getenv( 'REMOTE_ADDR' ));   //assume thelocal net is IP IV
        if ( true === WP_Http::is_ip_address( $ip ) ) {
           if ( '172' == $ip[0] && '10' == $ip[1] && '0' == $ip[2] ) return;   // change the parts of ip as required
        }
        $redirect = home_url() . '/wp-login.php?redirect_to=' . get_site_url;
        wp_redirect( $redirect );
     exit;
    }
}
add_action( 'wp', 'restrict_access_if_logged_out', 3 );