<P> A network switch is a device that forwards and filters OSI layer 2 datagrams (frames) between ports based on the destination MAC address in each frame . A switch is distinct from a hub in that it only forwards the frames to the physical ports involved in the communication rather than all ports connected . It can be thought of as a multi-port bridge . It learns to associate physical ports to MAC addresses by examining the source addresses of received frames . If an unknown destination is targeted, the switch broadcasts to all ports but the source . Switches normally have numerous ports, facilitating a star topology for devices, and cascading additional switches . </P> <P> Multi-layer switches are capable of routing based on layer 3 addressing or additional logical levels . The term switch is often used loosely to include devices such as routers and bridges, as well as devices that may distribute traffic based on load or based on application content (e.g., a Web URL identifier). </P> <P> A router is an internetworking device that forwards packets between networks by processing the routing information included in the packet or datagram (Internet protocol information from layer 3). The routing information is often processed in conjunction with the routing table (or forwarding table). A router uses its routing table to determine where to forward packets . A destination in a routing table can include a "null" interface, also known as the "black hole" interface because data can go into it, however, no further processing is done for said data, i.e. the packets are dropped . </P> <P> Modems (MOdulator - DEModulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless . To do this one or more carrier signals are modulated by the digital signal to produce an analog signal that can be tailored to give the required properties for transmission . Modems are commonly used for telephone lines, using a Digital Subscriber Line technology . </P>

State network topologies which is the best topology and why