A computer network is made up of many parts, and the longest-used method of conceptualizing computer networks is known as the seven-layer Open Systems Interconnection (OSI) model. For network technicians, the seven-layer OSI model provides a practical tool for diagnosing network problems, as well as a common language for describing networks.

Layer 1 – Physical: Includes the cable used to move bits of data between computer systems. Unshielded Twisted Pair (UTP) rated CAT 5E or CAT6 is common in most networks. Fiber optic cable that uses light waves to transmit data packets instead of electrical waves has significant speed and distance advantages over UTP, but lacks durability. Hubs, which function much like old-fashioned telephone exchanges that transfer data to connected systems, are also included in layer 1 of the OSI model, as are radio waves transmitted over the air to a wireless access point ( WAP).

Layer 2: Data Link – Includes the Network Interface Card (NIC), which serves as the interface between the computer and the network. The NIC is typically built into the motherboard and assigned a Media Access Control (MAC) address that gives each system attached to a network a unique ID. The MAC address is used to ensure data is delivered to the correct computer, and NICs use electricity to send and receive binary data packets such as pulses of electricity, light, or radio waves.

Layer 3: Network – Includes software designed to use logical addressing called a protocol that is used to move data packets efficiently across the physical and data link layers of the OSI model. The most common protocol is TCP/IP, which works in conjunction with routers at Layer 3 to ensure that data is sent to and received by the intended computer systems. Other protocols that use logical addressing like POP and SMTP for email also reside at the network layer.

Layer 4: Transport: includes a protocol designed specifically for assembling and disassembling data packets. Data must be sent in chunks that fit specific packet sizes, which are then reassembled by the receiving computer system into readable data. The transport layer network protocol partitions the data and assigns sequence numbers that allow receiving systems to process the data.

Layer 5: Session – Includes the protocol that initiates data transfer sessions between computer systems, accepts incoming sessions, and opens and closes existing sessions. The session layer network protocol also keeps track of computer naming conventions to facilitate data transfer sessions between computer systems.

Layer 6: Presentation – Includes software that converts data into standardized file formats that allow computers to receive data and convert it to a format that is readable by computer systems. PDF and Adobe Acrobat are an example of standardized file format software that allows computer systems to convert data into readable files.

Layer 7: Application – Includes software applications that allow users to access and use data files. Web browsers like Internet Explorer and Mozilla Firefox, as well as email programs like Outlook Express, are in the application layer. Layer 7 also includes code embedded in operating systems that enable network-aware applications and application programming interfaces (APIs).

Please don’t throw away the sweet potato: a useful mnemonic for memorizing the seven layers of the OSI seven-layer network model.