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Layers 3 and 4 — network and transport

With the advent of switches, bridges, and repeaters, a single network can become quite large. Nonetheless, we may want to divide such a network into two or more separate networks for organizational reasons (for example, separate networks for sales and marketing departments, or, in a SOHO environment, a Wi-Fi network for customers, and a separate internal network for the company). We may also want to connect to other people’s networks. In such cases, we need a network protocol.

It is commonplace to use the term network to refer to either [a] a network in the proper sense—a single network separated from other networks by a router—or [b] generically, to refer to the networked computers under our administrative control. When used in the latter sense, a network could actually refer to multiple networks using the former definition. For the sake of clarity, in this book, we will use the terms subnet, local network, or network segment to refer to a network in the former sense, and use the term network to refer to a group of network segments under the administrative control of a single entity.

The Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) protocol suite, which was introduced by Novell in 1983, gained a great deal of traction, primarily because it was the protocol suite used by Novell Netware, which was the most popular network operating system from the late 1980s through the mid-1990s. IPX is the network layer protocol, while SPX is the transport layer protocol, providing connection-oriented services between nodes.

As the internet grew in popularity, however, IPX/SPX began to lose ground to Transmission Control Protocol/Internet Protocol (TCP/IP), the protocol suite used on the internet. IP is the network layer of the suite, providing routing and addressing capabilities, and TCP is the transport layer of the suite, providing connection-oriented services and error-checking. TCP and IP are often discussed as if they are inseparable, but in cases where low latency is more important than reliability, IP can be paired with User Datagram Protocol (UDP), a connectionless protocol which does not provide error-checking capabilities. Today, TCP/IP is the dominant protocol suite for the network and transport layers.

You may have noticed that there is some redundancy in the OSI model—namely, that between layer 2 and layer 3, there are multiple ways of addressing a node. If you were wondering if there is a protocol that allows us to discover the link layer address associated with a network address, there is—it is called Address Resolution Protocol (ARP), and it is a layer 2 protocol. We can also obtain the network address if we have the link layer address using Reverse ARP.