Network models define a set of network layers (also known as a stack) and how those layers (protocols) interact with one another. There are two commonly used network models in use today, TCP/IP (Transmission Control Protocol/Internet Protocol) and OSI (Open Systems Interconnection). By using network models we enable devices using the same network model to communicate with one another more effectively and we’re better equipped to understand how the network works.

TCP/IP Model
The TCP/IP model is by far the most widely used model in existence today. Just about every modern networking device supports the TCP/IP model including computers, mobile devices, and even a few appliances. This model is also known as the DoD model and the Internet model. It was created by the efforts of the U.S. Department of Defense and various universities in the 1970s.

The TCP/IP model defines an enormous collection of protocols that are governed by their respective Request For Comments (RFC). It is human nature to categorize anything and everything to help us understand and remember. The TCP/IP model is no exception. It separates these protocols into four layers (or categories) to make it easier to understand how these layers (protocols) interact with one another. The four layers of the TCP/IP model are the Application, Transport, Internet, and Network Access layers with each layer providing a service to the layer above it.

The Application layer is the layer most of us will interact with on a daily basis whether we realize it or not. The protocols used in the Application layer provide services to the software running on a computer, such as retrieving the web page of your favorite website. The Application layer doesn’t care what the application looks like, what new features it has, what language it’s written in, or what you think of it. The Application layer defines what services the software might need, such as HTTP (web browsing) or POP3 and SMTP (e-mail). There are many protocols supported within the Application layer, such as HTTP, SMTP, POP3, FTP, Telnet, IRC, IMAP, SSL, PPP, etc.

The Transport layer on the other hand really comes down to two protocols: TCP and User Datagram Protocol (UDP). TCP is known as a connection-oriented protocol as it provides error recovery to data while UDP is known as connectionless as it doesn’t care about transmission errors. The Transport layer does exactly what its name implies: it transports data to and from the Application layer. It does this through the use of ports. The Transport layer provides message transfer capabilities regardless of the underlying network. It is the first layer in the TCP/IP model to provide reliability as well.

The Internet layer revolves around the IP protocol. This layer provides a few unique services to the network model. It provides an addressing scheme, known as an IP address, that every other device on the network is able to understand. This addressing scheme works the same way that the postal system’s address system works. The Internet layer also defines routing. Routing allows the data sent from one computer or network to arrive at the appropriate destination computer or network.

The Network Access layer (or Link layer) provides the protocols and hardware required to deliver data across a physical network. A common Network Access protocol is Ethernet (IEEE 802.3). It is the starting point for two devices to communicate across a network. The Network Access layer provides services to the Internet layer. It takes the data from the Internet layer and physically transmits it across the network. The required devices, cabling, addressing, and protocols are defined within the Network Access layer.

The TCP/IP model or stack is a collection of four layers that interact with one another to enable devices to communicate. Each layer, starting with the lowest level, the Network Access layer, provides a service to the layer above it.

OSI Model
The OSI reference model never succeeded in the marketplace but nevertheless contributed to many of the concepts and protocols currently used today. In the early days of computing and networking many vendors created their own proprietary network models. This presented a dire problem: how do devices manufactured by other vendors communicate with another vendors device? The International Organization for Standardization (ISO) took on this challenge in the late 1970s.

ISO created the OSI model using pre-existing protocols, standards, and conventions already used. They used the definitions and specifications of the technologies already in use and merely organized them into a network model for everyone to use. With the creation of TCP/IP and the OSI model, there was obviously some conflict as to which network model should be used. To keep a long story short, TCP/IP eventually won.

Why is the OSI model still important to us today? It provides a reference network model to compare and describe other network models to, hence the name the OSI reference model. The IP protocol is commonly referred to as a Layer 3 protocol and routers, which rely heavily on IP, are called Layer 3 devices.

The OSI reference model contains seven layers. They are, starting with the lowest layer: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Many of the layers present in the OSI model are present in the TCP/IP model and perform many of the same services and functions.

The Physical layer deals with the physical characteristics of the transmission medium, such as connectors, pins, use of pins, electrical currents, encoding, light modulation, and the rules for how to activate and deactivate the use of the physical medium. This layer is known as layer one.

The Data Link layer, known as layer two, is responsible for determining when a device can and cannot send data. It is often broken down further into two sub-layers known as the Logical Link Control (LLC) layer and Media Access Control (MAC) layer.

The Network layer or layer three behaves in much the same manner as the TCP/IP Internet layer.

The fourth layer, the Transport layer, provides the same services and functions as the TCP/IP Transport layer.

The Session layer, provides data error recovery and data flow control. It is also responsible for negotiating connections (sessions) between devices. The Session layer is layer five.

The Presentation layer, known as the sixth layer, is responsible for data presentation in the form of ASCII, BCD, EBCDIC, Binary, etc. This layer is also responsible for encryption.

The seventh and last layer of the OSI model, the Application layer, provides the same services and functions as the TCP/IP Application layer in addition to providing user authentication.

References: RFC 1122, RFC 1123