If you need to get information about users who are currently on a site, you can do this in two ways:

• Enable “Who’s online” standard block

• Create a custom block with the list of online users (their IPs and domain names)

Below, I will tell you how to create a custom block.

Create a custom block

The code below will show IP-addresses and domains of a site guests. The block appearance is similar to this one:

Drupal 5:

<?php
$number = db_result(db_query(‘SELECT COUNT(uid) AS number FROM {users} WHERE status=1′));
if (user_access(‘access content’)) {
// Count users with activity in the past defined period.
$time_period = variable_get(‘user_block_seconds_online’, 900);
// Perform database queries to gather online user lists.
$guests = db_fetch_object(db_query(‘SELECT COUNT(sid) AS count FROM {sessions} WHERE timestamp >= %d AND uid = 0′, time() – $time_period));
$guests_hostname = db_query(‘SELECT hostname FROM {sessions} WHERE timestamp >= %d AND uid = 0′, time() – $time_period);
$total_guests = db_num_rows($guests_hostname);
$users = db_query(‘SELECT uid, name, access FROM {users} WHERE access >= %d AND uid != 0 ORDER BY access DESC’, time() – $time_period);
$total_users = db_num_rows($users);
// Display a list of currently online users.
$max_users = variable_get(‘user_block_max_list_count’, 10);
if ($total_users && $max_users) {
$items = array();
while ($max_users– && $account = db_fetch_object($users)) {
$items[] = $account;
}
$output.=”<h7>Users</h7>”;
$output .= theme(‘user_list’, $items, NULL);
}
// Display a list of currently online guests.
if ($total_guests) {
$output.=”<div class=\”item-list\”><h7>Guests</h7><ul><fine>”;
$guestitems = array();
while ($guests– && $account = db_fetch_object($guests_hostname)) {
$guestitems[] = $account->hostname;
$output.=”<li><a title=\”Go to address\” href=\”http://$account->hostname\”>$account->hostname</a>
<a title=\”Go to address\” href=\”http://”.gethostbyaddr($account->hostname).”\”>”.gethostbyaddr($account->hostname).”</a>       “;
}
$output.=”</fine></ul></div>”;
}
}
return $output;
?>

Drupal 6:

To adjust this snippet to Drupal 6, you will need to replace db_num_rows() function with
mysql_num_rows() according to API  of mysql_num_rows() function.

Note: don’t forget to setup permissions to a block with this info. Users want to be anonymous – they may be disappointed if they see their personal info in a free access. Custom “Who’s Online” block

Good luck!