The next level in the OSI model above the physical layer is the data link layer. This layer is responsibile for creating frames that contain source and destination addresses, adding error detection and possibly correction fields to the frame, and, of course, incorporating a user’s data into the frame. Protocols at the data link layer are not routable, and examples of such layers are ethernet and token ring.
The layer where a network designer spends the most time is the network layer. This layer handles routing across the Internet and is the most important layer as far as multicasting is concerned. For a protocol to be routable, the addressing scheme must include a network and a host address. The last statement is true for “normal” IP traffic, but not for multicast traffic. As we will see, multicast addresses are not in the form of network/host but represent a group address. Although a network/host address pair is not present in a multicast address, multicast traffic is routable. Examples of routable protocols are IP, IPX, AppleTalk, and DECNet.
The transport layer is used to multiplex and demultiplex data streams between upper layer application processes as seen in Figure 1-2. The three upper layers of the OSI model, application, presentation, and session, have been combined in the application layer in the TCP/IP layered model. Typically, it is more difficult to determine where a particular upper layer application should be logically placed. Networks can be designed without knowing which applications the users are going to be employing. Therefore, the specific application is not important, just the protocol that the application will be using. In fact, we will only concern ourselves with the lower four layers of the OSI and TCP/IP models.
