Chapter 1: Network Models Flashcards
0What’s the 1st Layer of the OSI Model?
Physical Layer
Physical Layer Responsibility
Moving data between computers, no matter what it does.
Ways that physical layer moves data
Copper Cabling, Fiber Optics and Radio Waves
7 layers of the OSI Model
- Physical
- Data
- Network
- Transport
- Session
- Presentation
- Application
What is a NIC? What is its purpose?
Network Interface Card; it servers as the interface between the PC and the network.
What does “MAC” in MAC address stand for?
Media Access Control
Where are MAC addresses located
Inside special firmware that’s built into the ROM of the Network Interface Card.
How many bits are a MAC address
48
What notation are MAC addresses displayed in? Describe.
Hexadecimal; each character represents 4 bits, and 12 hex characters represent 4 bits.
What base does the hexadecimal numbering system use to represent numbers? What number range does this represent?
16; 0 - 15 in base 10 values.
How are hexadecimal bases represented?
Letter and number
If 0 - 9 are the first 10 characters in the hexadecimal system, what are the last five characters used?
A, B, C, D, E
What do the first six hex digits represent in a MAC address?
The number of the NIC manufacturer.
Who are the IEEE? What do they do?
Institute of Electrical and Electronics Engineers; provides a block of MAC addresses to NIC manufacturers.
What do the last six hex digits represent in a MAC address? What are these referred to as?
The manufacturers unique serial number; the Organizationally Unique Identifier (OUI).
Can anyone else use these MAC addresses?
No
What else are the last six digits in a MAC address referred to as?
The device ID
What’s another name for a MAC address?
Physical Address
What does MAC48 mean?
The MAC address is 48 bits, the first 24 define the OUI.
Unit of data specified by a protocol at each layer of the OSI 7 layer model.
Protocol Data Unit
Different frame types are used in
Different Networks
Why must all NIC’s on the same network use the same frame type?
So they can communicate with other NIC’s
What are the five things in a generic frame?
- Recipient’s MAC address
- Sender’s MAC address
- Type
- Data
- FCS
What is the “type” field in the generic frame?
what’s encapsulated in the frame.
What is the “data” field in the generic frame?
what’s encapsulated , followed by a special piece of checking information called “frame check sequence.”
What’s the Frame Check Sequence (FCS)?
A special piece of checking information that uses a type of binary math, that the receiving NI uses to verify that the data arrived intact.
What’s the binary math that FCS uses?
Cyclic redundancy check (CRC)
How many sections does a frame have? What are they?
3;
- Header
- Payload
- Trailer
What’s in the header of a frame?
MAC address and type
What’s in the payload of a frame?
Whatever’s encapsulated in the frame.
What’s in the trailer of a frame?
The Frame Check Sequence (FCS)
Which layer in the OSI model is the only layer that has sub layers?
Data Link
Why is the NIC at both Physical and Data Link Layers?
Physical - interface between network and pc
Data Link - sending and receiving
What’s the second layer of the OSI model?
Data Link Layer
Purpose of Data Link Layer
Nonde to node delivery of information; ensure error free data transmission of information.
What are the two aspects/jobs of NIC?
- Logical Link Control (LLC)
- Media Access Control (MAC)
What is Logical Link Control (LLC)? What does it do?
The aspect of the NIC that talks to the system’s OS (usually via drivers); handles multiple network protocols and provides flow control.
What does Media Access Control do?
4 things
- creates/addresses the frame.
- Adds the NIC’s own MAC and attaches the MAC address to the frames.
- The sub layer adds/checks the Frame Check Sequence
- Ensures that the frames are sent along the network cabling
What are the first two layers of the OSI model described as?
Network hardware
What are the third through seventh layers of the OSI model described as?
Network software
What is the third layer of the OSI model?
Network
Purpose of the network layer
Create and address packets so they can get sent from one network to another.
Any frame addressed specifically to another device’s MAC address is call a
Unicast frame
One to one addressing scheme is called
Unicast addressing
Windows uses this to separate characters in a MAC address, whereas Linux and macOS uses this to separate characters of its MAC addresses
Windows - a dash
Linux/macOS - a colon
Large networks need a
Logical addressing method
Why do large networks need a logical addressing method?
MAC addresses can’t be used anymore
What does logical addressing require to move past physical MAC addressing?
Network protocol
What does a network protocol do?
- Create unique identifiers for each system
- Create a set of communication rules for issue like how to handle data chopped into multiple packets
- How to ensure those packets get from one subnet to another
TCP/IP is several network protocols designed to work together, better known as a
Protocol suite
What does TCP stand for?
Transmission Control Protocol
What does IP do? How does it do it?
Makes sure that a piece of data gets to where it needs to go on the network.
Does this by giving each device on the network a unique numeric identifier called an IP address.
What is an IP address known as and why?
A logical address and to distinguish it from the physical address (MAC address).
What’s are the differences between an IP address and a MAC address?
- MAC is burned into the chip of the NIC; IP is stored in the systems software.
- MAC has part of its address identified by manufacturer; IP is identified by network.
- MAC characters are separated by dashes/colons; IP is separated by dots.
- MAC is known as IPv6; IP is known as IPv4
- MAC’s can’t be reused; IP’s can be reused
Number of containers data needs to be wrapped in for TCP/IP network to send data successfully.
What are they?
2
Frame - outer container
Packet - inner container
What does the frame do?
(TCP/IP - packets within frames)
Enables data to move from one device to another.
What does the packet do?
(TCP/IP - packets within frames)
IP-specific container that enables routers to determine where to send data, regardless of the physical connection type, and the data itself
This is created when each IP packet is handed to the NIC, which then encloses the IP packet in a regular frame.
Packet within a frame
What’s the fourth layer in the OSI model?
Transport Layer
What happens at the Transport layer?
Segmentation and Reassembly
What happens during Segmentation?
When a serving computer receives a request for some data, it must be able to:
1. Chop the requested data into chunks that will fit into a packet. Eventually into the NIC’s frame)
2. Organize the packets for the benefit of the receiving system
3. Hand them off to the NIC for sending
What happens during Reassembly?
Receiving system does this
- Must recognize a series of incoming packets as one data transmission
- Reassemble the packets correctly based on information included in the packets by the sending the system
- Verify that all the packets for that piece of data arrived in good shape
Why does Segmentation and Reassembly take place?
Most chunks of data are much larger than a single packet, and must be chopped before they can be sent across a network.
What else does the transport layer do?
Initializes requests for packets that weren’t received in good order.
Connection oriented protocol is
TCP
Connectionless protocol is
UDP
What does UDP stand for?
User Data Protocol
What’s left when you strip an IP address away from an IP packet?
TCP segment
What’s in a TCP segment?
- Source Port
- Destination Port
- Sequence Number
- Acknowledgement Number
- (and a bunch more)
- Data
A number between 1 and 65, 536, is a logical value assigned to specific applications or services.
Port
A number between 1 and 65, 536, is a logical value assigned to specific applications or services.
Port
Every TCP port contains this, so the client knows what to do with returning information.
Source Port
What does the Transport Layer do with data that comes from the Application layer?
- Breaks it into chunks
- Adds port and sequence numbers, creating the TCP segment
- Hands the TCP segment to the Network layer, this creates the IP packet
the Transport Layer does this after the TCP segment and IP packets are created.
Adds port and length numbers plus a checksum as a header, and combines with data to create a container called a UDP datagram.
Why doesn’t UDP care if the receiving computer gets its data?
It lacks most of the extra fields found in TCP segments.
What’s the fifth layer of the OSI model?
Session Layer
The system uses software to do what two things?
Segment and reassemble data packets.
Command to see how many sessions are running at one time.
netstat -a
Command to see a web browser’s session connecting to a web server.
netstat -n
If a system wants to send a print job to a printer, the system contacts the printer to see if it can handle a print job. What part of network does the session software handle?
Connecting applications to applications
What is the purpose of the Session Layer?
- Handles all the sessions for a system
- Initiates sessions
- Accepts incoming sessions
- Opens and closes existing sessions
What’s the sixth layer of the OSI model?
Presentation Layer
What does the Presentation Layer do?
Translates data from lower layers into a format usable by the Application layer, and vice versa.
What’s the messy part about the Presentation Layer?
TCP/IP networks don’t necessarily map directly to the OSI model.
What’s the seventh and final layer of the OSI model?
Application Layer
Why is it called the Application Layer?
Most visible part of any network, that refers to the code built into all Operating Systems that enable network-aware applications.
What’s API? What does it do?
Application Program Interface;
Provides a standard way for programmers to enhance or extend an application’s capabilities.
Encapsulation; what layers does it include? What do the layers do?
Process of preparing data to go onto a network.
Includes 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.
Decapsulation; what happens here?
Receiving computer reverses the process, stripping all the extra header information out as the data goes up the stack
What layers are involved in the decapsultation process? What do they do?
Transport - creates a segment or datagram and hands it down to the Network layer.
Network - adds IP information, encapsulating the segment or datagram.
Data Link - wraps up all that goodness, encapsulating the packet in a frame for delivery over the network.
NIC - turns the frame into bits and bytes for transmission over the network modem.
Client/server network. Give an example.
network that has dedicated server and client machines; user accesses server to do her job, laptop is a client of those servers.
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