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Networking and Internetworking with Bridges and Routers

More specifically, Ethernet LANs and IP networks occupy different layers of the Internet’s TCP/IP protocol suite. Between sender and receiver, networks deal with the bottom three layers of the model: the physical layer (Layer 1), the data link or MAC layer (Layer 2), and the network layer (Layer 3).

Note: These layers are also found in the Open Systems Interconnect Reference Model (OSI-RM); however, in this chapter they are applied to the TCP/IP protocol suite.

All digital networks ultimately deal with zeroes and ones, and the physical layer defines bit representation on the media. Physical layer standards also define mechanical aspects of the network, such as electrical characteristics or connector shapes, functional aspects such as bit sequence and organization, and so on. The physical layer only “spits bits” and has very little of the intelligence required to implement a complete network. Devices that connect LAN segments at the physical layer are called hubs, and all bits that appear on one port of the hub are also sent out on the other ports. This also means that bad bits that appear on one LAN segment are propagated to all other LAN segments.

Above the physical layer, the data link layer defines the first-order bit structure, or frame, for the network type. Also loosely called the MAC layer (technically, the MAC layer is a sublayer required only on LANs), Layer 2 sends and receives frames. Frames are the last things that bits were before they left the sender and the first things that bits become when they arrive on an interface. Because frames have a defined structure, unlike bits, frames can be used for error detection, control plane activities (not all frames must carry user data: some frames are used by the network to control the link), and so forth. LAN segments can be linked at the frame level, and these devices are called bridges. Bridges examine arriving frames and decide whether to forward them on an interface. All bridges today are called learning bridges because they can find out more about the network than could older bridges that were less intelligent devices. Bridges learn much about the LAN segments they connect to from protocols like those in the Spanning Tree Protocol (STP) family.

The network layer (Layer 3) is the highest layer used by network nodes to forward traffic as part of the data plane. On the Internet, the network layer is the IP layer and can run either IPv4 or IPv6, which are independent implementations of the same functions. The IP layer defines the structure and purpose of the packet, which is in turn the content of the frame at Layer 2. As expected, LAN segments (which now form perfectly functional networks on their own at the frame level) can be linked at the network layer, and in fact that is one of the major functions of IP. Devices that link LANs at the network layer are called routers, and IP routers are the network nodes of the Internet.