Networking Fundamentals Study Guide for the CompTIA Network+

General Information

On the CompTIA Network+ Exam, nearly one-fourth of the questions will involve knowledge of basic networking concepts. Fewer than one-eighth of those questions will begin with a scenario, so you must have a strong grasp of all the vocabulary and a good idea of when and where to employ solution strategies.

Open Systems Interconnection (OSI) Model

The OSI model, developed by the International Organization for Standardization (ISO), is the most frequently used reference model used for network communication design. You will need to be able to compare and contrast the seven layers of the OSI model as well as their corresponding encapsulation methods.

The Layers of the OSI Model

A reference model is a map or blueprint of how communications take place between devices. The OSI model was developed to create a standard communication pattern that could be used by all vendors to create an interoperable network by separating processes into logical groupings, or layers. The OSI model is composed of seven logical groupings known by both their layer number and their purpose.

Layer 1—Physical

Layer 1, or the physical layer, defines the physical topology of a network (how the physical cables and devices are arranged and connected). Logically, the physical layer receives and sends data in bits, which only transmit in binary, the base language of all computing devices. The physical layer sends or receives the binary and passes it up to the data link layer.

Layer 2—Data Link

Layer 2, or the data link layer, receives the bits from the physical layer and is responsible for the physical transmission of the data to the appropriate devices or locations through the use of hardware (or Ethernet) addresses, such as media access control (MAC) or logical link control (LLC) addresses. The data link layer also provides the network topology and is responsible for flow control. This layer takes the bits sent by the physical layer, combines them into bytes, and places them into data frames. A header is placed on the data frame by the data link layer, which contains the destination and source hardware addresses, and is then passed up to Layer 3r.

Layer 3—Network

Layer 3, or the network layer, is responsible for the logical device addressing on a network as well as their location and the most effective way of getting the data to where it needs to go. When the network layer receives the data frame, the header placed on the data is removed, and the logical destination address contained in the original data is evaluated and routed (in a packet) to the appropriate location, where it is passed up to Layer 4. The most common networking protocol is the Internet Protocol (IP).

Layer 4—Transport

Layer 4, or the transport layer, is responsible for turning received packets into a data stream via segments, which provides the logical connection between the lower three layers and the upper three layers. The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) are the most commonly used transport protocols.

Layer 5—Session

Layer 5, the session layer, receives the segments sent by Layer 4 and creates a separate communication connection between the transport layer and the presentation layer entities. The session layer is primarily responsible for keeping the data from one application separate from other applications.

Layer 6—Presentation

Layer 6, or the presentation layer, receives the data sent to it through the created session, which is still in binary format, and performs the data translation and code formatting, which is used by Layer 7. The presentation layer is also responsible for data compression, decompression, encryption, and decryption.

Layer 7—Application

Layer 7, or the application layer, takes the data that was translated by Layer 6 and uses it to create the user interface. The application layer is responsible for creating the connection between applications and the operating system (OS) via application processes, interfaces, or an application programming interface (API).

Data Encapsulation and Decapsulation

Encapsulation is the process of wrapping data with protocol data at each layer of the OSI model. Decapsulation is the reverse, where the wrapped protocol data is removed from the primary data bits. At each layer of the model, the encapsulation process creates a protocol data unit (PDU), which contains the protocol information added to the primary data. This data is typically found in a header placed before the data, but it can also be contained in a trailer at the end.

Ethernet Header

The Ethernet header is placed on the data at the data link Layer and contains the hardware addresses (MAC or LLC) of the source and destination, among other things. This PDU is referred to as a frame.

Internet Protocol (IP) Header

The IP Header is placed on the data at the networking layer and contains the logical addresses of the source and destination of the data. This PDU is referred to as a packet.

Other Headers

Headers created at the transport layer can be either TCP or UDP headers, which identify how data is transmitted between senders and receivers. Transport layer PDUs are referred to as segments.

Transmission Control Protocol (TCP) Header

TCP is a communication protocol that establishes a confirmed connection between senders and receivers prior to data transmission. The TCP data is contained in the TCP header created by the transport layer.

User Datagram Protocol (UDP) Header

UDP is a communication protocol that does not establish a confirmed connection prior to data transmission. The UDP data is contained in the UDP header created by the transport layer.

TCP Flags

A TCP flag, also known as a control flag, is a one-bit piece of data contained in the TCP header, which indicates the state of a TCP connection. There are nine possible TCP flags, with SYN, ACK, and FIN being the most commonly used.

Payload

The payload is the original data that needs to be transmitted through the network. The payload is encapsulated and decapsulated at each layer of the OSI model. The goal is to transmit the payload from the source to the destination with the payload intact.

Maximum Transmission Unit (MTU)

The MTU is the maximum size of a frame, packet, or segment in bytes or octets that can be transmitted across a data link. MTUs are limiters associated with Layers 2, 3, and 4 of the OSI model.