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 offer IP services into a home or business through many potential physical media (for instance, wireless, broadband, or baseband). This doesn't mean that when a network is designed one can ignore the lower two layers. It simply means that the physical media 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 relative silence exist during which very little or no bandwidth is needed. While email is simply one application, other applications running over IP follow similar burst tendencies (banking applications, video, voice over IP).
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.
An IP packet can be addressed 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, 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.