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. A K-line only disconnects a user from a single server, while 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 an extreme measure that's 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 many reasons why a user is banned from a network by a G-line. Basically, it happens when a user violates a policy of the network multiple times or becomes abusive to the network and other users.
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 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 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 user's identity protocol to be entered when they log onto the network. If the user is G-lined, the identity protocol appears as a match on the G-line list and the user is immediately be disconnected.
If the *@ghost option is used, the server performs a reverse DNS lookup to determine if the user's IP address is on the G-line list. This process takes more time, and may cause a delay before the ban takes effect. 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.