by Spencer Tiberi


  • We use the internet on a daily basis 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
        • May need an additional home router
          • Devices connect to a router via cables or wifi


  • 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 a IP address to your computer (router)
    • Used to be physically configured
    • DHCP (Dynamic Host Configuration Protocol)
      • Software that ISPs provides 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


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


  • 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 out 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 ISPs 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 to 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.)


  • IP goes beyond addresses
    • Set of conventions computers and servers follow to allow intercommunication
  • Fragmentation like in the envelope example are 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
  • TCP (Transmission Control Protocol) ensures packets can get to their destination
    • Commonly used with IP (TCP/IP)
    • Supports sequence numbers that help data get to its destination
      • When missing a packet, a computer can make a request for the missing packet
      • The computer will put packets together to get a whole file
    • Also includes conventions for requesting services (port identifiers)
      • To make sure Google knows we’re requesting a webpage and not an email or other service


  • Per TCP, the world has standardized numbers that represent different services
  • If is Google’s IP address,;80 (port 80) lets use know that we want a webpage
    • 80 means http (hypertext transfer protocol)
      • The language that web servers speak
    • Google will send the request to their web server via http
  • Many websites use secure connections with SSL or HTTPS, which uses the port 443
  • Email uses port 25
  • Other ports exist as well


  • Protocols are just sets of rules
    • Humans use these all the time, such as the protocol for meeting people: handshakes
  • When a request is made to Google for an image, HTTP tells Google how to respond appropriately


  • User Datagram Protocol
    • Doesn’t guarantee delivery
    • Used for video conferencing such as FaceTime
      • Packets can be dropped for the sake of keeping the conversation flowing
    • Used anytime you want to keep data coming without waiting for a buffer to fill

IPs in More Detail

  • IP addresses are limited
    • In the format #.#.#.#, each number is 8 bits, so 32 bits total
      • This yields 232 or about 4 billion possible addresses
        • We’re running out of addresses for all computers
    • Current version of addresses is IPv4
    • Moving towards IPv6
      • Uses 128 bits, yielding 2128 possible addresses
  • How do you find your IP address?
  • On a Mac, go to system preferences an poke around a bit

  • Private addresses exist
    • 10.#.#.#, 192.168.#.#, or 172.16.#.#
    • Only with special configuration can someone talk to your computer
    • Your personal device is not a server, so people should not need to access them directly
      • Your device needs to request data from servers
    • Even email is stored on a server such as Gmail and your device makes a request to that server to access that email
  • Looking at advanced settings…

    • Subnet mask is used to decide if another computer is on the same network
    • Router (aka Gateway) has its own address
      • Routs data in different directions
  • On windows:

    • Shows DNS servers as well


  • Routers have bunches if wires coming and going out of them
    • They have a big table with IP addresses and where data should be routed to get to that destination
      • Often, the data is routed to some next router
  • Routers purpose is to send data in the direction of a destination
    • The next router will send it to another until it reaches a destination

  • The internet is a network of networks (with their own routers)
    • Often multiple ways to go from A to B
      • Based in US Military logic to prevent downtime if a particular router goes down
      • When multiple packets are sent, like cat.jpg from Google, they can each take a different path, still getting to their destination eventually
        • Sometimes the internet is busy and the quickest path changes


  • How long does it take for this process of data transfer to take on the internet?
  • Traceroute is a program that sends packets to each router on a path to a destination, reporting the time it takes to reach that router
  • From Sanders Theatre to

    • 1-2: A few unnamed routers at Harvard
    • 3-4: More Harvard routers
    • 5-6: Level3 is a ISP
    • 7+: The routers are denying the request
  • From Sanders Theatre to

    • 6: Northern Crossroads
    • 7-14: A fast connection
      • 8-9: Chicago
      • 10-11: Denver
      • 12-13: Las Vegas
      • 14: Los Angeles
    • 19 is where it arrives at Berkeley in 80 ms!
  • From Sanders Theatre to

    • 6-7: Goes to New York connectivity
    • 8: MIT’s website is outsourced to Akamai’s NYC servers
  • From Sanders Theatre to

    • 9-10 jumps from Seattle to Osaka past an ocean!
      • Using undersea cabling

Undersea Cabling

  • David shows a video about undersea cables

Cable Modem Demo

  • David examines a home cable modem, focusing on its ports
    • Coaxial cable to plug into the wall
    • Phone jacks (RJ11) as many services are bundled together these days
    • Four jacks for ethernet cables (RJ45)
      • Devices can plug into these for internet connectivity
    • This modem has wifi support built in

Network Switch Demo

  • David examines a network switch
    • A device that you can plug into your router to allow more connections for all your other devices

Home Router Demo

  • David examines a home router
  • Home routers can have wifi, firewall, and switching capabilities

Network Cable Demo

  • David cuts open a network ethernet cable to examine its inner workings
  • Inside a network cable are 8 wires of different colors
    • Some are for transmitting data, others for receiving data
    • Others still are for insulation and cancellation of interference

Closing Thoughts and Homework

  • For homework, find a device that looks like a modem or router and take a look at the connectors on the back of it
    • If brave, play around with unplugging cables
      • Note: Your internet may go down in the process, but can be easily restarted with the cables properly reconnected!
    • If you have a spare ethernet cable, take a look inside yourself
      • These are a bit harder to put back together!