code is transmitted to the server, and it waits for the server to tell it whether the install should be permitted. The server maintains a database of all the serial numbers that have been used to install their software. If a single serial number is used on a number of machines (a typical limit would be five machines) then the server tells the software that it is likely a copy and to abort the installation. The users are usually presented with a dialog instructing them to contact the manufacturer. Watermarking Websites offering subscriber downloads may embed a digital watermark in the download, usually in a way that is not readily apparent to the user. For example, an identification number may be embedded in an image, or in metadata such as the date of a file. It is also possible to watermark multiple copies of a file with a unique watermark per recipient before sending them. In this case the embedded identification number can be the ID of the recipient. Other methods Some software that implements online registration extends this with a process commonly known as "phoning home". In this case, the software, either each time it is used or at some preset interval such as monthly, makes another connection back to the registration server. It does this to check in with the server to see if the serial number it is using has been determined to be one that is being used to install in many places. Serial numbers that have been identified as "pirated" (illegally distributed) are added to a blacklist on the server, a process referred to as being "burned". Burned serial numbers cannot be used to install or activate the product. Serial number lists are available on the internet that include a large number of valid registration codes for many software titles. It is common for software manufacturers to seek out these lists and invalidate the serial numbers that appear on these lists. This discourages individuals from giving out their registration codes for fear that this code will later be invalidated, disabling the original install of the software the next time that it "phones home". Some of the more expensive software requires the user to send personal information to the software vendor before receiving the activation code. The activation code is usually a large sequence of numbers and letters, and encodes information including the license serial number,