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«  B3. Why can't I mount more than 255 NFS file systems on my client? Why is it sometimes even less than 255?

   A. On Linux, each mounted file system is assigned a major number, which indicates what file system type it is (eg. ext3, nfs, isofs); and a minor number, which makes it unique among the file systems of the same type. In kernels prior to 2.6, Linux major and minor numbers have only 8 bits, so they may range numerically from zero to 255. Because a minor number has only 8 bits, a system can mount only 255 file systems of the same type. So a system can mount up to 255 NFS file systems, another 255 ext3 file system, 255 more iosfs file systems, and so on. Kernels after 2.6 have 20-bit wide minor numbers, which alleviate this restriction. »

http://www.makelinux.net/ldd3/chp-3-sect-2 :
« Within the kernel, the dev_t type (defined in <linux/types.h>) is used to hold device numbers—both the major and minor parts. As of Version 2.6.0 of the kernel, dev_t is a 32-bit quantity with 12 bits set aside for the major number and 20 for the minor number. Your code should, of course, never make any assumptions about the internal organization of device numbers; it should, instead, make use of a set of macros found in <linux/kdev_t.h>. »
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