A G-line, also known as a global kill line, is a global network ban. It's similar to a K-line or kill line in that a user is banned from a network. The major difference between a G-line and a K-line is that while a K-line only disconnects a user from a single server, a G-line disconnects the user from an entire network. A user usually receives a G-line after he or she has received a K-line on another server and continues to abuse the network from a different server.
A G-line is considered an extreme measure that is used as a last resort when all previous methods of attempting to discipline an abusive user have failed. This also means that only high ranking global IRC operators are usually permitted to set G-lines, especially on larger networks. K-lines are usually reserved for local affairs and are handled by operators of individual servers.
There are numerous reasons why a user might be banned from a network by a G-line, and these reasons can vary from one network to the next. Basically, it happens when a user violates a policy of the network multiple times or becomes abusive to the network and other users.
A G-line can also last for any period of time. Some G-lines are in place for only a short period, which means the user has to wait for it to wear off. Other G-lines can be put into place for a longer period of time, and if the abuse is severe enough a G-line can be put into place permanently.
A G-line can be stored in the configuration file of an IRCd, but some network operators prefer to store them in the service's configuration files instead. If a G-lined user attempts to access the IRC network from which they were banned, they are automatically disconnected. Sometimes they may receive a message stating the reason for the ban, but they may also merely be disconnected.
Despite their similarities to K-lines, G-lines do work a little differently from other kill lines. A G-line is typically set as either *@IPaddress or *@ghost, but the first option is usually preferred. When the *@IPaddress option is used, the G-line waits for the identity protocol of a user to be entered, which of course occurs when said user logs onto the network. If the user has been G-lined, the identity protocol will appear as a match on the G-line list and the user will immediately be disconnected.
If the *@ghost option is being used, the server performs a reverse DNS lookup on the user to determine if his or her IP address is on the G-line list. It is a slower process that often results in a delay between when the user tries to log onto a network and when he or she is banned. It is also why this method is not the preferred method. The user does get disconnected from the network, but he or she can still get onto the network for a short time.