In general, segmentation is the process of breaking something into smaller units. In networking terms, segmentation is the process in which a large data is broken down to a number of small-sized data units. This is performed by the sender before transmitting the packets. With reference to Open Systems Interconnection (OSI), the segmentation is performed at the transport layer. The small-sized data units are called Protocol Data Units (PDUs). The network layer will encapsulate PDUs into packets later down the path towards hardware.

Large sized packets undergo segmentation to enable transmission of packets through the network. This is because of packet size restriction offered by the network.

There are five classes of transport layer protocols, ranging from Transport Protocol Class 0 (TP0) to Transport Protocol Class 4 (TP4). TP0-TP3 works only with connection-oriented communications whereas TP4 works with both connection-oriented and connectionless communications. All these classes perform segmentation during transmit.

The protocol at the transport layer (for example, TCP) determines the smallest maximum PDU size supported by the network. Data from the upper layers undergo segmentation to PDUs of the size so determined.

Reassembly
All such received PDUs will be reassembled back to form the original data at the receiving end; and the process is termed reassembly. This is again performed at the transport layer of the OSI.

There is some processor overhead for both segmentation and reassembly of huge data. Some network interface cards (NICs) and associated drivers provide segmentation facility either at the physical layer or at the driver layer. This is called offloading and with this it is possible to improve the throughput.

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