limited range, its signal can be received by other, nearby, garage door openers. When two neighbors had garage door openers, then opening one garage door might open the neighbor’s garage door as well. The second stage of the wireless garage door opener system solved the opening-the-neighbor's-garage-door problem. The remote controls on these systems transmitted a digital code, and the receiver in the garage responded only to that code. The codes were typically set by eight to twelve DIP switches on the receiver and transmitter, so they allowed for to different codes. As long as neighbors used different codes, they would not open each other's garage doors. The intent of these systems was to avoid interference with nearby garage doors; the systems were not designed with security in mind. Intruders were able to defeat the security of these systems and gain entry to the garage and the house. The number of codes was small enough that even an unsophisticated intruder with a compatible remote control transmitter could just start transmitting all possible codes until he found one that opened the door. More sophisticated intruders could acquire a black box master key that automatically transmitted every possible code in a short time. An even more sophisticated method is known as a replay attack. The attacker would use a code grabber, which has a receiver that captures the remote's digital code and can retransmit that digital code at a later time. The attacker with a code grabber would wait nearby for the homeowner to use his remote, capture the code, and then replay the code to open the door when the homeowner was gone. Multicode openers became unpopular in areas where security was important, but due to their ease of programming, such openers are often used to operate such things as the gates in gated apartment complexes. An intermediate stage of the garage door opener market eliminated the DIP switches and used remotes preprogrammed to one out of roughly 3.5 billion unique codes. The receiver would maintain a security list of remotes to which it would respond; the user could easily add the unique remote's code to the list by pressing a button on the garage door opener while activating the remote control. A large number of codes made the brute force try-all-possible-digital-codes attacks infeasible, but the systems were still vulnerable to code grabbers. For user convenience, these systems were also backward compatible