|  | 3.3 | How to Identify and Select Networking Capabilities
| 3.3.4 | Describe the three-layer design model components
A layer is identified as a point in the network where an OSI reference model Layer 3 (network layer) boundary occurs: The three layers are bound by Layer 3 devices or other devices that separate the network into broadcast domains. As shown in the figure, the three-layer model consists of core, distribution, and access layers, each of which has specific functions: Core layer -- The core layer provides fast wide-area connections between geographically remote sites, tying a number of campus networks together in a corporate or enterprise WAN. Core links are usually point-to-point, and there are rarely any hosts in the core layer. Core services (for example, T1/T3, Frame Relay, SMDS) typically are leased from a telecom service provider. Distribution layer -- The distribution layer gives network services to multiple LANs within a WAN environment. This layer is where the WAN backbone network is found, and it is typically based on Fast Ethernet. This layer is implemented on large sites and is used to interconnect buildings. Access layer -- The access layer is usually a LAN or a group of LANs, typically Ethernet or Token Ring, that provide users with frontline access to network services. The access layer is where almost all hosts are attached to the network, including servers of all kinds and user workstations. A three-layer model can meet the needs of most enterprise networks. However, not all environments require a full three-layer hierarchy. In some cases, a two-layer design may be adequate or even a single layer flat network. Even in these cases, however, a hierarchical structure should be planned or maintained to allow these network designs to expand to three layers as the need arises. The following sections discuss in more detail the functions of the three layers. Then, we'll move on to discuss one- and two-layer hierarchies.