A network bridge
If someone says about bridge then an average user imagines huge building that connecting the two banks of the river and sometimes a lot of speeding cars. In all, it can visualize operation of the bridge on the network colloquially. The bridge is a device which connects two or more networks or network segments by making the filtering of network traffic. Network connected to the bridge can use various physical and logical link protocols. Bridge essentially works in the second OSI layer of the computer network. On the basis of the recipient`s address the bridge can decide where the data will be sent. It can choose the correct route and optimize the data transfer. It is a device that analyses packets at the data link layer of the ISI/OSI model. During the work, it examines the self-created forwarding array. It includes port numbers (E0/0, E0/1, etc.). They are connected to one device and the MAC hardware addresses. Bridges operate in the promiscuous mode. They receive data which are circulating in the transmission medium. To determine which devices are in different network segments (associated with specific ports), bridges read the source MAC address of data frames. On this basis, it creates forwarding table (in free translation "bridging table"). Despite the fact that (some of) the bridges are not visible to other devices (since it does not modify frames, only be "eavesdropping"), the use of the bridge increases the network latency by 10-30 percent (but at the same time the network load can be reduced). This follows from the decision that the bridge must take before forwarding the packet. The bridge is considered as preserving and transmitting device (called store and forward). Before sending the frame further, the bridge examines field containing the address and the code calculating the CRC specified in the code field of the control frame. If the destination port is busy, the bridge temporarily retains the frame until it releases.