In directory listings, the major device number is always shown first, followed by the minor device number. Every device on the system has both major and minor device numbers assigned in such a way as to ensure that they are unique. If two devices are assigned the same number, Linux can't properly communicate with them. You can't create a device number within the device driver or by numbering the files. You must use mknod every time you want to configure a new device on your system.
Some devices use the major and minor device numbers in a strange way.
Some tape drives, for example, use the minor number to identify the density of the tape in order to adjust their output. These types of exceptions are rare, luckily, and don't occur on most Linux systems. You do not have to create these types of device numbers because an installation script usually handles the setup of the device drivers and their numbers. The -p option lets you set file permissions. One of these arguments must be present on the command line for the device number to be properly assigned.