The major number for the hd (IDE) driver is hard-coded at 3. The minor numbers have the format

(<unit>*64)+<part>

where <unit> is the IDE drive number on the first controller, either 0 or 1, which is then multiplied by 64. That means that all hd devices on the first IDE drive have a minor number less than 64. <part> is the partition number, which can be anything from 1 to 20. Which minor numbers you will be able to access will depend on how many partitions you have and what kind they are (extended, logical, etc.). The minor number of the device node that represents the whole disk is 0. This has the node name hda, whereas the other device nodes have a name equal to their minor number (i.e., /dev/hda6 has a minor number 6).

If you were to have a second IDE on the first controller, the unit number would be 1. Therefore, all of the minor numbers would be 64 or greater. The minor number of the device node representing the whole disk is 1. This has the node name hdb, whereas the other device nodes have a name equal to their minor number plus 64 (i.e., /dev/hdb6 has a minor number 70).

If you have more than one IDE controller, the principle is the same. The only difference is that the major number is 22.