OSI Model

Common basis for the coordination of standards development for the purpose of system interconnection.

#Layer 1 - Physical

• Computer data exists in the form of bits (ones and zeros)
• Something has to transport those bits between hosts.
• Cables (ethernet twisted pair, coaxial, fiber), Wifi, Repeaters, Hubs (multiport repeaters)

#Layer 2 - Data Link (Hop to Hop)

• Interacts with physical layer (put bits on the wire and retrieve bits from the wire)
  • NIC - Network Interface Card
  • Wifi access cards
  • Switches
• Addressing Scheme - MAC addresses
  • 48 bits, represented as 12 hex digits
  • Every NIC has a unique MAC address

#Layer 3 - Network (End to End)

• Addressing Scheme - IP Addresses
  • 32 bits, represented as 4 octets, each 0-255
  • Routers, Hosts, anything with IP

#Layer 4 - Transport (Service to Service)

• Distinguish data streams
• Addressing Scheme - Ports
  • 0-65535 - TCP (favors reliability)
  • 0-65535 - UDP (favors efficiency)
• Servers listen for requests to pre-defined ports
• Clients select random port for each connection
  • Response traffic will arrive on this port

#Layer 5,6,7 - Session, Presentation, Application

• Application protocols

#Examples

#OSI Model and Internet

#SSL Opinion 1

#SSL Opinion 2

#Physical: Ethernet