When using IP addresses as a means to block unwanted visitors, it must be maintained.
The Shield Security plugin does this automatically based on the time that has passed since a blocked IP address has visited the site.
We want the IP address lookup queries for each visitor to be fast and as optimized as possible and so we automatically clean out old IP address from the list.
An IP Address is considered "old" when the time that has elapsed since their last access on the site exceeds the time you specify as the "Auto Expire" interval.
A user gets blocked because of too many transgressions. They try to visit the site and they get blocked. Your auto expire is set to 1 day. If that same visitor accesses within 24hrs, we update the last access time and the 1 day counter starts again from zero. They must wait 1 day and 1 second before trying to access the site again.
The Auto Block Expiration option settings
Permanent and lengthy IP Black Lists are harmful to performance. You should allow IP addresses on the black list to be eventually removed over time. Shorter IP black lists are more efficient and a more intelligent use of an IP-based blocking system.
To set the automatic expiration timeout, simply chose one of the options from the drop-down list:
More information about the auto-expiration of black listed IP addresses can be found here.
Note: Block Bad IPs/Visitors module also gives you the ability to manage blacklisted and whitelisted IP addresses. To find out how this IP Lists Manager works, read the article here.