Fragmentation is a state that sometimes occurs when data passes over networks. The advent of the Internet means that information may cross many networks to reach its destination. Those networks are beyond the control of the system sending the data and the different sections of the journey may involve adapting to incompatible network settings. Data packets may have to be split up, or “fragmented” in order to comply with the requirements of carrying networks.
When data crosses a network it is split up into segments. This prevents one connection monopolising the resources of the network. Each segment has to reach its destination and be reassembled at the receiving end. The direction, control and reassembly of data requires instructions to pass over the network along with the segments of data. These instructions are tacked on to the front of the data and the structure contains the admin data together with the data is called a “packet.”
The outer header on a packet follows standards set out in the “Internet Protocol.” IP includes parameters that include a maximum packet size. If a packet goes out over the Internet it must cross other networks. If that network is set to receive a maximum packet size (called a Maximum Transmission Unit) that is smaller than the size of the arriving packet, the receiving router splits the packet up and adds its own headers to enable the packets to be reassembled at the receiving end. Fragmented packets do not get reassembled after they leave the limiting network, even though the next network may allow packets of the original size.
The act of fragmenting a packet slows its journey slightly. As all the packets in a transmission are created by the same source, it is likely that all of those packets will be too long for a carrier network, rather than just one packet in the stream. This slows down the transmission considerably. The packets will also have to be reassembled into their original form at the receiving computer, which slows reception procedures.
You may read about fragmentation on your computer. This is not the same form of fragmentation and not a setting for the MTU. In fact the fragmentation that a computer’s operating system refers to is the existence of gaps in data storage where previously stored data has been erased. You do not adjust the MTU of your network by “defragmenting” your computer.