Internet Protocol

IP itself is a connectionless protocol that resides at Layer 3 (the network layer), which means that no reliability mechanisms, flow control, sequencing, or acknowledgments are present. Other protocols, such as TCP, can sit on top of IP (Layer 4, session) and can add flow control, sequencing, and other features.

Given IP's relative position in the OSI reference model, it doesn't have to deal with common data link issues such as Ethernet, Asynchronous Transfer Mode (ATM), Frame Relay, and Token Ring, or with physical issues such as Synchronous Optical Network (SONET), copper, and fiber. This makes IP virtually ubiquitous.

You can run IP into a home or business through any means necessary (for instance, wireless, broadband, or baseband). This doesn't mean that when you design a network you can ignore the lower two layers. It only means that they are independent of any applications you put on IP.

IP is considered a bursty protocol, which means that the applications residing above IP experience long periods of silence, followed by a need for a large portion of bandwidth. A good example of this is e-mail. If you set your mail package to download e-mail every 20 minutes, about 20 minutes of silence exist during which no bandwidth is needed.

One of the major benefits of IP is the ability to write an application once and have it delivered through an assorted type of media anywhere, regardless of whether this occurs through a digital subscriber line (DSL) connection in your home or a T1 line in your business.

You can address an IP packet in three general ways: through unicast, multicast, or broadcast mechanisms. Briefly explained, these three mechanisms provide the means for every IP packet to be labeled with a destination address, each in its unique way:

• Unicast is fairly simple, in that it identifies one specific address and only that node is supposed to send the packet to the higher layers of the OSI reference model.

• Broadcast packets are sent to all users on a local subnetwork. Broadcasts can traverse bridges and switches, but they are not passed through routers (unless they are specially configured to do so).

• Multicast packets use a special addressing range that enables a group of users on different subnetworks to receive the same flow. This enables the sender to send only one packet that several disparate hosts can receive.

Unicast, broadcast, and multicast packets each have a significant purpose. Unicast packets enable two stations to communicate with each other, regardless of physical location. Broadcast packets are used to communicate with everyone on a subnetwork simultaneously. Multicast packets enable applications, such as videoconferencing, that have one transmitter and multiple receivers.

Regardless of the type of IP packet used, data link layer addressing is always needed. Data link layer addresses are covered in detail in the next section.

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