Chapter 1: Network Models

Question 1

List each layer (7 to 1) of the OSI Seven-Layer Model.

Answer 1

L7 Application
L6 Presentation
L5 Session
L4 Transport
L3 Network
L2 Data Link
L1 Physical

Remember the mnemonic “All People Seem To Need Data Processing.”

Question 2

Define: Protocols

Answer 2

Sets of clearly defined rules, regulations, and procedures that enable hardware and software developers to make devices and applications that function properly at a particular network model layer.

Question 3

What does a NIC or Network Interface Card do and what network layer does it operate in?

Answer 3

A NIC serves as the interface between the PC and the network. It operates at L2 and uses MAC addresses.

Question 4

Define: MAC or Media Access Control Address (aka. a physical address)

Answer 4

A 48-bit (6-byte) unique identifier that is burned onto a ROM chip in every NIC. It is in hexadecimal notation (12 hex digits).

Question 5

What are the two parts of a MAC address?

Answer 5

The first six hex digits represent the NIC manufacturer, referred to as the Organizationally Unique Identifier (OUI).

The last six hex digits are the manufacturer’s unique serial number for that NIC, referred too as the device ID.

Question 6

In Windows, type ______ from a command prompt to display the MAC address.

Answer 6

ipconfig /all

Question 7

In macOS, type ______ from a terminal to display the MAC address.

Answer 7

ifconfig

Question 8

In Linux, type ______ from a terminal to display the MAC address.

Answer 8

ip a

Question 9

Define: Protocol Data Unit (PDU)

Answer 9

It is the unit of data specified by a protocol at each layer of the OSI seven-layer model.

Question 10

What is the PDU for Layer 2, the Data Link layer?

Answer 10

Frame (Ethernet)

Question 11

What does a frame do and what creates and uses them?

Answer 11

A frame encapsulates information and data for easier transmission. The NIC creates and sends, as well as receives and reads frames.

Question 12

What are the first two fields of a generic frame?

Answer 12

Recipient’s MAC address
Sender’s MAC address

Question 13

What is in the Type field of a frame?

Answer 13

The Type field indicates what’s encapsulated in the frame.

Question 14

What is in the Data field of a frame?

Answer 14

The Data field contains what’s encapsulated.

Question 15

What is the purpose of the Frame Check Sequence (FCS) field of a frame?

Answer 15

The FCS uses a type of binary math called a Cyclic Redundancy Check (CRC) to calculate a value that the receiving NIC uses to verify that the data arrived intact.

Question 16

What is the max size of an Ethernet frame’s Data field?

Answer 16

1500 bytes

Question 17

What is the size of the FCS field?

Answer 17

4 bytes

Question 18

What is the Layer 2 broadcast address?

Answer 18

FF-FF-FF-FF-FF-FF

Question 19

Who receives a Layer 2 broadcast address?

Answer 19

Every single NIC on the network except the sending NIC.

Question 20

What is contained in a Layer 2 broadcast address?

Answer 20

A request for a system’s MAC address, by the use of that system’s known IP address.

Question 21

What are the two sublayers of the Data Link layer?

Answer 21

Logical Link Control (LLC)
Media Access Control (MAC)

Question 22

What is the job of the Logical Link Control (LLC) sublayer?

Answer 22

It is the part of the NIC that talks to the system’s operating system (usually via device drivers), and handles multiple network protocols and provides flow control.

Question 23

What is the job of the Media Access Control (MAC) sublayer?

Answer 23

It creates and addresses the frame, adds or checks the FCS, and sends the frame along the network cable.

Question 24

What is a switch and at what layer does it operate.

Answer 24

A switch is a network device that handles network traffic using MAC addresses to direct the traffic. It operates at Layer 2.

Question 25

What is the PDU for Layer 3, the Network layer?

Answer 25

Packet

Question 26

What is the protocol used in the Network layer?

Answer 26

Internet Protocol (IP)

Question 27

What does the Internet Protocol do?

Answer 27

It gives each device on the network a unique numeric identifier called an IP address (logical address).

The Internet Protocol is the primary logical addressing protocol for TCP/IP.

Question 28

What is an IP address?

Answer 28

An IP address is 32-bits (4 bytes) long. It uses dotted decimal notation based on four 8-bit numbers. Each byte is separated by a decimal (192.168.4.232).

Question 29

What is a router and at what layer does it operate?

Answer 29

A router is a networking device that connects subnets and networks and forwards data using IP addresses. It operates at Layer 3 (Network).

Question 30

Explain how packets and frames work.

Answer 30

As a frame travels, if it goes through routers, each router strips off the incoming frame, determines where to send the data according to the IP address in the packet, creates a new frame to encapsulate the packet, and then sends it all on its way to its next stop. The new frame type will be the appropriate technology for whatever connection technology connects to the next router. The IP packet remains unchanged.

Question 31

What are the PDUs for the Transport Layer?

Answer 31

Segments (TCP) and Datagrams (UDP)

Question 32

What is Transmission Control Protocol (TCP), including what Layer it operates?

Answer 32

TCP is a connection-oriented protocol at Layer 4. It breaks up data into chunks called segments that will fit into packets, and gives each segment a sequence number.

Question 33

What does the Transport Layer do?

Answer 33

It is the segmentation/reassembly software, that breaks up data into segments to send and reassembles data from segments that it receives. It also initializes requests for packets that weren’t received in good order.

Question 34

What logical addressing is used in the Transport Layer?

Answer 34

Port addressing

Question 35

What is a port?

Answer 35

A port - a number between 1 and 65,535 - is a logical value assigned to specific applications or services.

Question 36

What is User Datagram Protocol, including what Layer it operates at?

Answer 36

UDP is a connectionless protocol at Layer 4. It creates datagrams with less fields than TCP segments because it doesn’t care whether the receiving computer gets its data.

Question 37

What 5 fields are in a UDP datagram?

Answer 37

Source Port
Destination Port
Length
Checksum
Data

Question 38

What does the Session Layer do?

Answer 38

It uses session software to connect applications to other applications between computers/devices by creating sessions.

It handles all the sessions for a system by initiating sessions, accepting incoming sessions, and opens and closes existing sessions.

Question 39

What does the Presentation Layer do?

Answer 39

The Presentation Layer translates data from lower layers into a format usable by the Application Layer, and vice versa.

Question 40

What is the Application Layer?

Answer 40

The Application Layer refers to the code built into operating systems that enables network-aware applications.

All operating systems have Application Programming Interfaces (APIs) that programmers can use to make their programs network aware.

Question 41

What is encapsulation?

Answer 41

The entire process of preparing data to go onto a network. This includes all the steps from the Application, Presentation, Session, Transport, Network, and Data Link layers. Each layer adds more information so that the data gets to the correct recipient and the recipient knows what to do with the data.