Introduction

• We use the internet daily and have constant access and connectivity

• Home network

  

  • Cable modem, DSL modem, or FIOS device
    • Connects to the internet
    • Pay monthly for an ISP (Internet Service Provider)
      • Verizon, Comcast, etc.
    • Could have built-in wireless connectivity for your devices
      • We may need an additional home router
        • Devices connect to a router via cables or wifi

IP

• Every computer on the internet has an IP (Internet Protocol) address
  • Of the form #.#.#.#
    • Four numbers separated by dots of the values 0-255
    • Other IP address formats exist today as well
  • Like postal addresses, they uniquely identify computers on the internet
    • Any device connected to the internet has an IP address
      • Allows other computers to talk to it
• ISPs assign an IP address to your computer (router)
  • Used to be physically configured
  • DHCP (Dynamic Host Configuration Protocol)
    • Software that ISPs provide to allow your computer to request an IP address
    • DHCP servers respond with a specific IP address for your Home
  • Multiple devices can connect to your home network
    • The home router supports DHCP and assigns IP addresses to your devices

DNS

• We access websites using domain names (Facebook.com, Google.com, etc.), but it turns out that these sites to have IP addresses
• DNS (Domain Name System) servers convert domain names into IP addresses

Packets

• Computers communicate by sending packets, which are like virtual envelopes sent between computers

  • Ultimately still 0s and 1s

• As an analogy, suppose we want to find a cat image on the internet

• So, we send a request to a server, say Google, like “get cat.jpg”

  • We place this request in an envelope

• On the envelope, we list our IP as the return address

• However, for the recipient of the request, we don’t know the IP address for Google

  • Have to rely on DNS
  • Send a request to our ISP’s DNS server for Google’s IP address
    • If the ISP’s DNS server doesn’t know a website’s IP address, it has been configured to ask another DNS server
    • There exist root servers that know where to look for an IP address if it exists

• After sending the request off, we’ll get a response ms later

  

• The cat will be sent back in one or more packets

  • If the cat image is too large for a single envelope, sending it in one packet could take up internet traffic
  • To solve this, Google will divide the cat image into smaller fragments
    • Put the fragments into different envelopes
    • Write information on the envelopes
      • Return address: Google’s IP address
      • Delivery address: Our IP address
      • List the number of packets on each envelope (1 of 4, 2 of 4, etc.)

TCP/IP

• IP goes beyond addresses
  • Set of conventions computers and servers follow to allow intercommunication
• Fragmentation like in the envelope example supported by IP
  • If missing a packet, you can logically infer which packet you’re missing based on the ones received
    • However, IP doesn’t tell computers what to do in this case