MBR Partition Types: Primary, Extended, and Logical Partitions

Learn how MBR partition tables organize primary, extended, and logical partitions, including numbering rules, Linux device names, capacity limits, and GPT differences.

What a partition table does

A partition table is disk metadata that records how storage space is divided into independently addressable regions called partitions. The operating system uses this information to find each partition's boundaries, type, and related attributes.

A disk and a partition are different things. The disk is the complete storage device; a partition is a region defined within that device. A filesystem can then be created inside a partition and mounted for use by the operating system.

MBR, or Master Boot Record, is a legacy partition-table format commonly associated with BIOS-era x86-compatible computers. Modern systems often use GPT instead, but MBR remains important when maintaining older computers, removable media, virtual machines, or legacy boot configurations.

The Master Boot Record

The MBR is located at the beginning of a disk. Historically, this area had two related jobs:

• It could contain boot-startup code used by a BIOS-based system.
• It contained the disk's compact partition table.

The MBR partition table has exactly four entry positions. Each position can describe a primary partition, or one position can describe an extended partition. This fixed number of entries is the source of MBR's four-entry limitation.

MBR partition types

MBR uses three commonly discussed partition types: primary, extended, and logical. Primary and extended partitions use the direct entries in the MBR table. Logical partitions are created inside an extended partition.

Primary partitions

A primary partition is represented directly by one of the four MBR partition-table entries. An MBR disk can therefore contain up to four primary-partition entries.

Primary partitions are numbered from 1 through 4. For example, a disk may have:

• Partition 1: a system or boot partition
• Partition 2: a data partition
• Partition 3: another data partition
• Partition 4: a recovery partition

When all four entries are used by primary partitions, no entry remains for an extended partition. As a result, that disk cannot use logical partitions unless its layout is changed.

Extended partitions

An extended partition is a special partition-table entry that acts as a container. It uses one of the four primary-entry positions, but its purpose is to hold logical partitions rather than to serve as an ordinary filesystem-bearing data partition.

Only one extended partition can exist on an MBR disk. That single container can provide space for multiple logical partitions, allowing the disk to have more usable volumes than the four direct MBR entries would otherwise permit.

The extended partition itself receives a partition number from 1 through 4 because it occupies a direct MBR entry. Its logical partitions receive numbers starting at 5.

Logical partitions

A logical partition is a partition created inside the extended partition. Logical partitions provide additional usable volumes beyond the direct primary-entry limit.

Logical partitions require an extended partition. They cannot exist independently on an MBR disk. Logical partitions are normally the partitions that receive filesystems and are mounted for use.

Logical partition numbering begins at 5. This numbering convention applies even when some of the numbers from 1 through 4 are unused. For example, if partition 1 is primary and partition 2 is extended, the first logical partition is still commonly identified as partition 5, not partition 3.

How the partition types relate

An MBR disk can use either only primary partitions or a mixture of primary partitions and one extended partition containing logical partitions.

• Four primary partitions use all four direct MBR entries.
• Three primary partitions plus one extended partition use all four direct entries.
• One primary partition plus one extended partition uses two direct entries, leaving the extended container available for logical partitions.

The key distinction is the relationship between the entries. A primary partition is directly described by the MBR table. A logical partition is nested inside the extended partition and is reached through the extended-partition structure.

Example: four direct partitions

In this layout, every MBR entry describes a primary partition:

MBR entry 1  - primary partition 1
MBR entry 2  - primary partition 2
MBR entry 3  - primary partition 3
MBR entry 4  - primary partition 4

This arrangement leaves no direct entry for an extended partition, so no logical partitions can be added without changing the layout.

Example: two primary partitions and logical partitions

Here, partition 3 is an extended container. Partitions 5, 6, and 7 are logical partitions inside it:

Partition 1  - primary system partition
Partition 2  - primary data partition
Partition 3  - extended container
  Logical 5  - filesystem-bearing volume
  Logical 6  - filesystem-bearing volume
  Logical 7  - filesystem-bearing volume

The extended partition consumes one of the four MBR entries even though the useful filesystems are stored in its logical partitions.

Linux device naming and partition numbers

On a traditional SATA or SCSI-style Linux disk named /dev/sda, primary and extended entries are commonly shown as /dev/sda1 through /dev/sda4. Logical partitions are commonly shown as /dev/sda5, /dev/sda6, and so on.

Modern device names can look different. An NVMe disk may be named /dev/nvme0n1. Its partitions include an additional p separator, such as:

/dev/nvme0n1p1  - partition 1
/dev/nvme0n1p5  - partition 5, commonly a logical partition on an MBR layout

The partition number indicates its position in the MBR numbering scheme. It does not indicate importance, performance, filesystem type, or whether the partition is bootable. Bootability depends on the partition metadata, firmware mode, boot loader, and operating-system configuration.

Inspecting an MBR layout in Linux

Use read-only inspection commands before making partition changes. Output varies by device and system configuration.

Using fdisk

sudo fdisk -l

This lists disks and partitions. Its output can show whether a disk uses an MBR-style partition table and how its entries are numbered.

Using parted

sudo parted -l

This reports partition-table types. In its output, msdos generally identifies an MBR partition table, while gpt identifies GPT.

Using lsblk

lsblk -o NAME,SIZE,TYPE,FSTYPE,PARTTYPE

This presents block devices and partitions in a compact hierarchy. It is useful for relating device names to sizes, filesystems, and partition types.

MBR disk-size limitation

Traditional MBR addressing uses a limited number of addressable sectors. With conventional 512-byte logical sectors, this produces an approximate maximum addressable capacity of 2 TiB.

TiB means tebibyte, based on powers of two: 1 TiB is 2⁴⁰ bytes. Storage manufacturers commonly advertise capacity in decimal units, where 1 TB is 10¹² bytes. Therefore, the often-reported “2 TB MBR limit” is an approximation of the binary boundary near 2 TiB. The exact practical result also depends on sector size, firmware, operating-system support, and the partitioning tools involved.

For a disk larger than the traditional MBR range, an MBR layout may leave part of the disk inaccessible or prevent the full capacity from being allocated. GPT, or GUID Partition Table, is generally preferred for modern large-capacity disks.

MBR compared with GPT

MBR can remain appropriate when compatibility with old firmware, operating systems, boot loaders, or existing tools is more important than newer features. For a new installation on a large disk, especially with UEFI firmware, GPT is normally the better choice.

Troubleshooting common MBR issues

A tool will not create a fifth primary partition

The disk already uses all four MBR partition-table entries. Use an existing or newly created extended partition for logical partitions, or move to GPT when the hardware, firmware, operating system, and backup plan support that change.

The first logical partition is numbered 5

This is normal. MBR reserves partition numbers 1 through 4 for direct primary-table entries, including an entry used for the extended partition. Logical numbering starts at 5 even if some lower numbers are not used.

A large disk cannot be fully allocated

The disk may exceed traditional MBR addressing capacity under the 512-byte-sector assumptions. Check the partition-table type and disk size, then consider GPT after confirming boot-mode and operating-system compatibility.

An extended partition cannot be formatted or mounted normally

An extended partition is primarily a container, not an ordinary data volume. Create and format logical partitions inside it, then mount those logical partitions.

A user expects multiple extended partitions

MBR permits only one extended partition entry. Put the required logical partitions inside that single container or use GPT instead.

Exam-relevant summary

• The MBR is disk metadata at the beginning of a disk and historically combines boot-startup code with a partition table.
• The MBR partition table has four direct entries.
• A primary partition uses one direct MBR entry and is numbered 1 through 4.
• An extended partition uses one direct MBR entry and acts as a container.
• Only one extended partition can exist on an MBR disk.
• A logical partition exists inside the extended partition and is numbered 5 or higher.
• The extended partition counts toward the four-entry maximum.
• Traditional MBR layouts with 512-byte logical sectors have an approximate 2 TiB capacity boundary.
• GPT is generally preferred for new large disks and modern UEFI-focused installations.

For a related reference within this subject, see MBR partitions.