Mục đích của một router là hai lần: • Để chuyển tiếp lưu lượng truy cập có nghĩa là cho các địa điểm xa xôi hẻo lánh dọc theo một lộ trình phù hợp, do đó nó sẽ đến đúng địa chỉ. • Để ngăn chặn lưu lượng truy cập hoàn toàn địa phương bị rò rỉ ra ngoài mạng nội bộ và gây ách tắc không cần thiết. | . NETWORKS 51 LANs WANs and VLANs In the 1980s and 1990s most networks consisted of a hierarchy of routers joined into a Wide Area Network WAN . Each Local Area Network or local community such as a business or university would have its own gateway router connecting it to the rest of the world. The purpose of a router was two-fold To forward traffic meant for remote locations along a suitable route so that it would arrive at the right address. To prevent purely local traffic from leaking out of the local network and causing unnecessary congestion. When an electrical signal passes along a cable it is like a light being switched on in a room. The picture of a network transmission as a stream of bytes travelling along a cable like cars in a train is often In local area networks the distances are often so short that transmission is almost instantaneous and each bit fills an entire cable segment though this depends on the data rate. Every bit every 1 or 0 is a signal a voltage or light pulse on a cable which fills a space the size of a wavelength at about two-thirds of the speed of light in a vacuum - so on short segments this is often the entire cable. It is like sending Morse code with a lighthouse. Every part of the network sees the signal but only the addressed recipient normally bothers to read it. Figure Traffic control with a router. Routers forward traffic that needs to leave a local network and shield the external world from local traffic. A router isolates one part of a network from another both logically and It will only forward the signal if the signal needs to travel along another segment to reach its destination address see figure . The router is able to make this determination based on information about the topology of the network. This is an important function in the network if every signal sent by 4In conventional encoding schemes a single bit is represented by one wavelength of the basefrequency clock rate. .
