245DX-600/800APR 2002Edition 1.07.3.4. RouterRouters, operating at the OSI Network layer, organize the large network in terms of logical networksegments. Each network segment is assigned an address so that every packet has both a destinationnetwork address and a destination device address.Routers are more intelligent than bridges. Not only do routers build tables of network locations, but theyalso use algorithms to determine the most efficient path for sending a packet to any given network byidentifying its header information.These are the typical functions:1. RoutingThis controls the traffic according to a specified routing table.2. Packet FilteringThis performs the access and security control for specified routing.3. Address ConversionNAT (Network Address Translator), This performs conversion of a single global IP Address from/tosingle private IP Address.4. IP Masquerade:This performs a conversion of single global IP Address from/to multiple private IP Address.At the same time the port number is automatically assigned.Occasionally, the conversion creates a bottleneck in the network overhead. For a typical solution, PIX(Private address Internet address exchange) is available from Cisco, which is a well-knownmanufacturer.5. Designated ReplyThese are reply that keep a connection alive by responding with a signal periodically.Watch Dog in IPX/SPX, TCP/IP in TCP, and Net BT (NetBIOS on TCP/IP) in Windows NT are all wellknown techniques to keep a live connection.PC-A192.168.32.1/24Router BPC-B192.168.32.2/24PC-C192.168.33.1/24PC-D192.168.33.2/24access-list permit tcp 192.168.32.0 0.0.0.255 192.168.33.1 0.0.0.0 eq 23OKOKpermission deniedpermission denied192.168.32/24Packet Filtering Sample192.168.33/24
