How come two devices share the same major-minor device number?

I am reading "Linux device drivers, 3rd edition", and found something i can't understand.

in Chapter 3.2, the author said:

Traditionally, the major number identifies the driver associated with the device. The minor number is used by the kernel to determine exactly which device is being referred to.

Then I tried "ls -l /dev" to have a look, I found some thing unusual:

brw-rw----  1 root disk      1,   1 201开发者_JAVA技巧1-08-23 23:52 ram1
brw-rw----  1 root disk      1,   2 2011-08-23 23:52 ram2
brw-rw----  1 root disk      1,   3 2011-08-23 23:52 ram3
brw-rw----  1 root disk      1,   4 2011-08-23 23:52 ram4
brw-rw----  1 root disk      1,   5 2011-08-23 23:52 ram5
...
crw-r-----  1 root kmem      1,   1 2011-08-23 23:52 mem
crw-r-----  1 root kmem      1,   4 2011-08-23 23:52 port
crw-rw-rw-  1 root root      1,   3 2011-08-23 23:52 null
crw-rw-rw-  1 root root      1,   5 2011-08-23 23:52 zero

These devices (ram1-ram5) all have a clone, with the same major-minor, but different name and type. I thought the author was saying "major number means device class, and minor number means device index. So Major-Minor identifies a unique device."

Now I am confused. How come two devices could share the same major-minor? What exactly the device numbers are?

Correct me if I was wrong.. Thanks in advance.

mem, port, null, and zero are character devices (as evidenced by the c the listing starts with). The ramN devices are block devices (hence the b). Major/minor numbers for block devices are independent from character devices, and vice versa.