Overview

Sections:

• Physical Layer
• Data-Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

Overview: 

• Routing principles and fundamentals are a critical part of networking as its purpose is to ensure that data packets are properly carried between different devices, subnets or networks and often across multiple routers

Use Case Scenarios:

• Efficient Data Path Selection
  • Routing helps in determining the best path for data to travel from the source to the destination ensuring the data arrives quickly and reliably 
• Network Scalability
  • Routing enables large networks to scale by connecting multiple networks and managing the flow of data between them
• Fault Tolerance and Redundancy
  • Routing allows for dynamic route adjustments (alternate paths) in cases where network failures occur
• Network Segmentation
  • Routing allows networks to be segmented into subnets or sub networks making it easier to manage, secure, and troubleshoot large networks

How it Works:

To better understand routing, let's discuss the function of a Layer 3 routing device by taking a look back into all 7 layers of the OSI model and comparing the layers with the Postal Service Analogy.

Layer 1: Physical

OSI Model Definition:

• In OSI standards, the physical layer consists of the physical components for connecting devices into the physical network including but not limited to:
  • Copper cables (Cat5/Cat6) / Electrical signals / RJ45 connectors / RJ45 SFPs / PoE Injectors
    • 
  • Fiber Optic cables / Light signals / Fiber connectors / Fiber SFPs
    • 
  • Wireless Transmission / radio waves 
    • 
  • Network Interface Cards (NICs)
    • 

Postal Service Analogy:

• In the Postal Service Analogy, the physical layer refers to the paper component on which the letter is written

Layer 2: Data Link

OSI Model Definition:

• In OSI standards, the data link layer consists of the Ethernet frames and the MAC addresses between devices handling the transfer of data correctly over the physical medium
  • Switches operate at the Data Link layer to connect devices within a LAN to forward frames based on MAC addresses
    • 

Postal Service Analogy:

• In the Postal Service Analogy, the data link layer refers to the envelope component housing the letter

Layer 3: Network Layer

OSI Model Definition:

• In OSI standards, the network layer is responsible for the routing, forwarding, and addressing of data packets across different networks
  • Routers operate at the network layer examining the destination IP address of incoming packets to forward them according to the best available route
    • 

Postal Service Analogy:

• In the Postal Service Analogy, the network layer refers to the postal address on the envelope to determine the route the letter will take to reach the destination

Layer 4: Transport Layer

OSI Model Definition:

• In OSI standards, the transport layer provides end to end communication between source and destination devices to ensure the data is delivered across the network
  • TCP and UDP layer 4 protocols are used to handle the flow of these communications and utilize different port numbers to the data to identify which application the data belongs to
    • 

Postal Service Analogy:

• In the Postal Service Analogy, the transport layer refers to postal service ensuring the letter is delivered correctly

Layer 5: Session Layer

OSI Model Definition:

• In OSI standards, the session layer is responsible for establishing, maintaining, and terminating communications between two devices or applications in a network
  • Once the communication is complete, the session layer terminates the session, ensuring proper closure of the communication between the two devices or applications

Postal Service Analogy:

• In the Postal Service Analogy, the session layer refers to the scheduling and coordination of sending multiple letters by the postal office

Layer 6: Presentation Layer

OSI Model Definition:

• In OSI standards, the presentation layer is responsible for dealing with data formatting, encryption, and decryption 
  • Ensures that data transferred from one system's Application layer can be read by the Application layer of another system

Postal Service Analogy:

• In the Postal Service Analogy, the presentation layer refers to the translation services required for international mail

Layer 7: Application Layer

OSI Model Definition:

• In OSI standards, the application layer is responsible for representing the actual data being transmitted and interacts directly with the end user's application and provides network services

Postal Service Analogy:

• In the Postal Service Analogy, the application layer refers to the content of the letter itself