====== Information ======

  * 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

====== TTL ======

  * 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

====== Encapsulation ======

  * 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