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  • 5-layer network model description
  • For Coursera The Bits and Bytes of Computer Networking week 3

Physical Layer

  • Lowest level
  • Where data transfers via electrical currents, voltage, radio frequency, and modulation takes place
  • A simple problem at this layer could be an Ethernet cable being severed or unplugged from a network switch

Data link layer

  • Handles error-correction with data packets received from the physical layer
  • Handles transferring data packets received from the physical layer, to network interfaces via MAC addresses

Network layer

  • Routing occurs in this layer, where packets need to be transferred to their destination
  • Packets may have to travel between many different network switches in large-scale networks (like the internet) depending on the destination's physical location
  • Packets hopping to each router has the TTL header subtracted from

Transport layer

  • Various protocols (commonly TCP and UDP) are specified and used here
  • TCP is used here and operates on a response-based system where packets received need to be acknowledged
  • UDP is used here and does not need packets to be acknowledged
  • TCP is good for reliability, whereas UDP is good for speed and lower latency
  • TCP is typically used for web server and file transfer connections
  • UDP is typically used for game server and VPN connections

MAC address

  • Identifier assigned to the network interface of a device
  • All network interfaces have an unique MAC address
  • A MAC address does not change depending on where the network interface is connected
  • Composed of hexadecimal digits (0-9, A-F, and a-f), normally in groups of two separated by - or :
  • Exists at data link layer

IP address

  • Address assigned to a device on a network
  • Addresses can be assigned dynamically (DHCP) or statically based on the network the interface is connected to
  • IP address can change depending on the network the network interface is connected to
  • IPv4 is the most common version at 32-bits of data, and represented in dot-decimal notation
  • The total number of possible IPv4 addresses are exhausted at large-scale
  • IPv6 is gaining popularity, is 128-bits of data, and contains a significantly larger pool of available addresses than IPv4

TCP port

  • Defined in the Transport layer
  • Is an endpoint that defines what a service is that is sending or receiving packets
  • Popular services have well-known port assignments, such as 80 for HTTP, 443 for HTTPS, 21 for FTP, and 6112 for various game servers
  • Typically referred to in firewall applications, and can be used to block services on a specific device, or network-wide

Checksum check

  • When the data in a payload section is checked to verify its contents

Routing table

  • Defines where network interfaces are
  • Exists on most network switches, including consumer routers
  • Normally automatically maintained and updated by the network switch


  • A header in an IP packet that exists at the Network layer
  • Defines how many hops a packet is allowed to perform
  • When a packet transfers from a network switch to another network, the TTL header is subtracted from by one
  • Used to prevent mis-configured packets from endlessly looping between devices


  • Data is encapsulated at various points in the network layer
  • The data that exists in a TCP or UDP (or any protocol at the network layer) packet is encapsulated as the data payload in the IP datagram
  • The headers specific to an IP datagram, and the data payload that exists in it are then encapsulated as the payload for an Ethernet frame
coursera/network_model.txt · Last modified: 2019/02/11 03:32 by Sean Rhone