Following the code area, the MBR stores data about four partitions, known as primary partitions. Each partition is described in two ways: using cylinder/head/sector (CHS) notation and using logical block addressing (LBA) notation. The CHS notation is almost a historical footnote today, because it's a 24-bit number. This means that it's limited to describing areas of about 8GB in size. The 32-bit LBA values permit 2TiB sizes, assuming a sector size of 512 bytes. This 2TiB ceiling is not easily overcome; there simply aren't any unallocated fields left in the MBR that could be used to add more bits to the LBA addresses.
In addition to the looming 2TiB problem, the MBR presents other difficulties. Chief among these is the limitation of four primary partitions. To work around this limitation, it's possible to set aside one primary partition as a placeholder (known as an extended partition) to hold an arbitrary number of additional partitions, known as logical partitions. This is, however, an ugly workaround that creates its own problems, such as difficulties installing multiple operating systems when too many of them want too many primary partitions to themselves.
