TCP/IP & Protocols

• Order of TCP/IP stack:

  Application, transport, network, link.
• What does application layer do?

  Provides protocols for communicating between devices
• What does the transport layer do?

  Uses TCP to establish end-to-end connection with recipient computer
• What does the network layer do?

  Uses IP to address packets with the source and destination IP address
• What does a router do at the network layer?

  Forwards each packet to an endpoint, defined by IP address and port number - called a socket.
• What does the link layer do?

  Adds the MAC address of NIC at destination IP address
• What happens to the MAC address at each hop?

  It changes.
• When receiving data what happens to the TCP/IP stack?

  It is done in reverse
• When receiving data:

  The link layer removes MAC address from each packet

  The Network layer removes the IP address from each packet

  The Transport layer removes the port number from each packet and reassembles packets into the correct order

  The application layer presents the data for the recipient online
• What type of data are MAC addresses?

  Hexadecimal
• What does a MAC address do?

  It uniquely identifies a device with a NIC
• A port is used to alert specific applications to deal with data sent to a computer
• Port numbers:

  20 = FTP data

  21 = FTP control

  22 = SSH

  25 = SMTP

  80 and 8080 = HTTP

  143 = IMAP

  443 = HTTPS
• FTP is a protocol at the application layer to move files across a network
• What is SSH?

  An encrypted protocol that allows secure communication between nodes across a network
• How is SSH secure?

  SSH uses public key encryption to protect the data during communication
• What is a common use of SSH?

  Remote management of servers and other computers
• What is SSH tunnelling?

  SSH can be used to bypass a firewall by sending packets through the SSH port (port 22) to send data that might otherwise be blocked
• IMAP stands for Internet Message Access Protocol, which is an email access protocol.
• SMTP is used to send emails and forward them between mail servers and their destination
• POP3 downloads email stored on a remote server to a local client.

  Emails are removed after downloaded
• IMAP manages emails on a server so multiple clients can access them simultaneously
• Web pages are written in CSS, HTML and JavaScript
• Web servers are dedicated computers that host websites and provide resources on request
• HTML, CSS and JavaScript are separated and broken down into their constituent parts. This is then further broken down and rendered according to a standard hierarchal model.
• Web page resources such as images are downloaded as the web page is rendered
• DHCP is a protocol that automatically assigns IP addresses from a pool of available addresses for a certain amount of time.
• What is the order of the DHCP process?

  • Client broadcasts DHCP Discover message
  • Server replies with a DHCP Offer message
  • Client broadcasts DHCP Request message
  • Server replies with a DHCP Ack message