<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> <P> A firewall is a network device for controlling network security and access rules . Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones . The vital role firewalls play in network security grows in parallel with the constant increase in cyber attacks . </P> <P> The study of network topology recognizes eight basic topologies: point - to - point, bus, star, ring or circular, mesh, tree, hybrid, or daisy chain . </P>

Who is topology does not allow transferring data directly from computer to computer