A hub is a network device that connects various network nodes, for example, in an Ethernet network, in a star configuration. In the OSI (Open Systems Interconnection) reference model, hubs are classified as level 1 devices that operate at the physical layer. Their main task is to interconnect multiple computers and forward received data immediately. Hubs are usually made of plastic, run on external power, and contain between 4 and 16 ports, which are the physical connections. Their maximum bandwidth is typically 10/100 Mbit per second.
The hub receives the data and sends it completely to all connected devices (hosts). All ports of the hub run at the same speed and are in a collision domain. Unlike other network devices, hubs do not provide the option to control or exclude only individual receivers. This means that during transmission, all packets are always forwarded to all computers. This means that even devices for which the data was not intended will receive the data. Since all hosts are occupied in this way, other devices cannot send any data on their own during this time. Instead, concurrent requests will be processed one after another.
If more hosts are needed, one center can be connected to another center. This connection is created with a simple crossover cable through one of the ports. However, the number of hosts is automatically limited by the 5-4-3 rule or the repeater rule. This means that up to five segments with four repeaters can be used between two end devices. Additionally, when using a hub, the connected hosts share the entire bandwidth. This inevitably results in a loss of speed, especially when transmitting large packets.