o create 'FORWARD' the packet onto the extended network. If the destina- one logical extended network out of two, (or more), locally dis- tion device address is on the same network segment as the source persed LANs. device, then the bridge will not forward the packet; instead it will block its path onto the extended network effectively keeping it local. In this way the bridge acts as a 'FILTER' of data packets. 56.5.2.1 Local bridges By means of 'Filtering and Forwarding' bridges can create one Bridges operate at the Data Link Layer, (Layer 2), of the OSI Model, single logically unified network out of several locally discrete or more specifically at the Media Access Control, (MAC), sublayer. LANs, while at the same time limiting the flow of unnecessary (See Figure 56.18.) traffic between them. BRIDGE Figure 56.17 LAN interconnection using a local bridge Internetworking devices 56/13 56.5.3 Routers DATA LINK DATA LINK LAYER LAYER The second solution to the problem of interconnecting LANs over unlimited geographical distances was the router. PHYSICAL PHYSICAL This device operates at the Network Layer of the OSI Model, LAYER LAYER (Layer 3). Routers essentially offer selective routing of individual packets over multiple communications paths. (See Figure 56.20.) MEDIUM NETWORK NETWORK Figure 56.18 OSI model of a bridge LAYER LAYER DATA LINK DATA LINK Organisations that had local area networks within their geographi- LAYER LAYER cally dispersed operating sites soon began to express a desire to interconnect all their remote LANs together, to effectively build a PHYSICAL PHYSICAL single organisation-wide area network, irrespective of geography. LAYER LAYER Manufacturers responded to this requirement with two types of solutions, remote bridges and routers. MEDIUM Manufacturers offered as a simple, 'transparent' solution, bridges that incorporated wide area interfaces to provide access to different types of media, such as leased or switched analog telephone lines, 48, 56, 64kbit/s lines, or E-1 or T-1 lines. Figure 56.20 