CSCI E-1a

Internet

by Spencer Tiberi

Introduction

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

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 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

DNS

We access websites using domain names (Facebook.com, Google.com, etc.), but it turns out that these sites too 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 âget cat.jpgâ
- 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 donât know the IP address for Google
- Have to rely on DNS
- Send a request to our ISPs DNS server for Googleâs IP address
  - If the ISPâs DNS server doesnât know a websiteâs 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, weâll 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: Googleâs 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 are supported by IP
- If missing a packet, you can logically infer which packet youâre missing based on the ones received
  - However, IP doesnât 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 weâre requesting a webpage and not an email or other service

Ports

Per TCP, the world has standardized numbers that represent different services
If 5.6.7.8 is Googleâs IP address, 5.6.7.8;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

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

UDP

User Datagram Protocol
- Doesnât 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
    - Weâre 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 settingsâ¦
- 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

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

Traceroute

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 Google.com:
- 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 Berkeley.edu
- 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 MIT.edu
- 6-7: Goes to New York connectivity
- 8: MITâs website is outsourced to Akamaiâs NYC servers
From Sanders Theatre to CNN.jp
- 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!