License plate look-ups are done through the Department of Motor Vehicles, usually through one of their state websites or by asking at one of their office locations. Some states do not offer a free license plate search service, requiring individuals to contact the police in order to locate plate numbers.
Although the DMV keeps license plate records, they are limited as an organization by state laws. They cannot provide license plate numbers to citizens of certain states. They can, however, check for duplicates when creating a custom license plate number. For many states, this is as close as the DMV can get to providing license plate information.
License plate information is tracked by the police, but since it is personal information, it is sketchy to keep track of it. Police primarily use license plate information to track down automobiles belonging to suspects, or to find stolen vehicles.
License plate numbers have personal information associated with them, such as addresses and names. This information can be used to cause serious problems for the car's owners, which is why there are laws and measures to protect the license plate owner's information. However, if someone already has this information, or the car's VIN number, they can use online services to find the license plate number itself.