What are valid UID numbers for use in /etc/passwd?

Tim Rohaly

A User ID, or UID, may take on the values of 1 through 255 inclusive. UID of 0 is reserved for the use of "guest", and may not be specified as an argument to the adduser command.

A UID greater than 127 grants that user superuser privilege. The default root account has a UID of 128.