Here is a query you can run on your database to retrieve a list of all users that have used a particular IP address:
Code:
SELECT user.username, INET_NTOA(ip.ip)
FROM xf_ip AS ip
LEFT JOIN xf_user AS user ON (user.user_id = ip.user_id)
WHERE ip.ip = INET_ATON('xxx.xxx.xxx.xxx')
GROUP BY user.user_id
Replace xxx.xxx.xxx.xxx with the IP you want to search for.
SELECT user.username, INET_NTOA(ip.ip)
FROM xf_ip AS ip
LEFT JOIN xf_user AS user ON (user.user_id = ip.user_id)
WHERE ip.ip BETWEEN INET_ATON('xxx.xxx.xxx.1') AND INET_ATON('xxx.xxx.xxx.255')
GROUP BY user.user_id
You have to specify the beginning and end of the range.