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Network Infrastructure

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Summary

PDF p.114

Network infrastructure is analyzed using a layer model, such as the OSI model. It consists of nodes and links, with nodes being either hosts or intermediaries. Hosts initiate data transfers, while intermediaries forward traffic. Networks are categorized as LANs or WANs based on their scope. Addressing and forwarding functions are managed by various network appliances and protocols across different OSI layers.

In plain words

Supplementary — not from your PDF

Networks are made of nodes (hosts that start transfers, and intermediaries that forward traffic) and links. The OSI model splits the work into layers. Switches and access points use MAC addresses at layer 2, routers use IP addresses at layer 3, TCP/UDP ports identify applications at layer 4, and application protocols such as HTTP and DNS work at layer 7.

Detailed explanation

PDF p.114
  • Layer Model
    • OSI Model: Defines layers of network functions.
    • Physical (PHY) Layer (Layer 1): Links implemented as twisted-pair cables, fiber optic cables, or wireless devices.
  • Nodes and Links
    • Host Nodes: Initiate data transfers (servers or clients).
    • Intermediary Nodes: Forward traffic around the network.
    • Network Scope
      • Local Area Network (LAN): Single site.
      • Wide Area Network (WAN): Metropolitan, country-wide, or global scope.
  • Addressing and Forwarding
    • Unique Addressing: Each network node must have a unique address.
    • Different Layers and Scopes: Addressing functions occur at different layers with different scopes.
  • Network Appliances and Protocols
    • Switches
      • Function: Forward frames between nodes in a cabled network.
      • Layer: Work at layer 2 of the OSI model.
      • Addressing: Use MAC addresses (48-bit value in hexadecimal notation).
      • Broadcast Domain: Addressing works within the local network segment.
    • Wireless Access Points
      • Function: Bridge between cabled network and wireless hosts.
      • Layer: Work at layer 2 of the OSI model.
      • Addressing: Use MAC addressing.
    • Routers
      • Function: Send packets around an internetwork.
      • Layer: Work at layer 3 of the OSI model.
      • Default Gateway: Acts as a default gateway for hosts to send packets to other segments.
    • Transport Protocols
      • TCP: Establishes reliable connections.
      • UDP: Allows unreliable, connectionless transfers.
      • Layer: Defined at layer 4 of the OSI model.
      • Ports: Each application protocol identified by a TCP or UDP port.
    • Application Protocols
      • Function: Support client/server functionality for user-level services (e.g., web browsing, email, file transfer).
      • Layer: Work at layer 7 of the OSI model.
    • Domain Name System (DNS)
      • Function: Host name records and perform name resolution.
      • Layer: Works at layer 7 of the OSI model.
      • Service Type: Infrastructure service, not user-level service.

Important terms

taken from the text above
OSI Model
Defines layers of network functions.
Physical (PHY) Layer (Layer 1)
Links implemented as twisted-pair cables, fiber optic cables, or wireless devices.
Host Nodes
Initiate data transfers (servers or clients).
Intermediary Nodes
Forward traffic around the network.
Local Area Network (LAN)
Single site.
Wide Area Network (WAN)
Metropolitan, country-wide, or global scope.
Unique Addressing
Each network node must have a unique address.
Different Layers and Scopes
Addressing functions occur at different layers with different scopes.
Layer
Work at layer 2 of the OSI model.
Addressing
Use MAC addresses (48-bit value in hexadecimal notation).
Broadcast Domain
Addressing works within the local network segment.
Default Gateway
Acts as a default gateway for hosts to send packets to other segments.
TCP
Establishes reliable connections.
UDP
Allows unreliable, connectionless transfers.
Ports
Each application protocol identified by a TCP or UDP port.
Service Type
Infrastructure service, not user-level service.
PHY Physical LAN Local Area Network WAN Wide Area Network DNS Domain Name System

Examples & real-world scenarios

Supplementary — not from your PDF
  • A switch forwarding frames using 48-bit MAC addresses.
  • A router acting as the default gateway to other subnets.
  • DNS resolving a host name to an IP address.

Scenario

A laptop can reach printers in its own office but nothing beyond. The likely cause is a missing or wrong default gateway (a layer 3 router setting), not a switch problem.

Common mistakes

Supplementary — not from your PDF
  • Putting DNS at layer 3 because it deals with addresses. It is an application-layer (layer 7) infrastructure service.
  • Mixing up MAC addresses (layer 2, local segment) and IP addresses (layer 3, across networks).

Practical skills

Supplementary — not from your PDF
  • Match a device or protocol to its OSI layer and addressing type.

What I should remember

Key Points PDF p.114
  • Layer Model
    • OSI Model: Defines network functions.
    • Physical Layer: Twisted-pair cables, fiber optic cables, wireless devices.
  • Nodes and Links
    • Host Nodes: Servers or clients.
    • Intermediary Nodes: Forward traffic.
    • Network Scope: LANs and WANs.
  • Addressing and Forwarding
    • Unique Addressing: Different layers and scopes.
  • Network Appliances and Protocols
    • Switches: Layer 2, MAC addresses, broadcast domain.
    • Wireless Access Points: Layer 2, MAC addressing.
    • Routers: Layer 3, default gateway.
    • Transport Protocols: TCP (reliable), UDP (unreliable), layer 4.
    • Application Protocols: Layer 7, user-level services.
    • DNS: Layer 7, infrastructure service.