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<WRAP syllabus round>

===== Week 10 — The Internet =====

This week's topic is the Internet: what it is and how it works.

==== Evaluation ====

Up to 10 points can be gained towards your final score by completing the
**in-class quiz** on Friday.

==== What you will learn from this class ====

  * What the Internet is.
  * What a Network Interface Card is and how NICs are identified by their MAC addresses.
  * How wires, cables, and WiFi connect devices together to form a Local Area Network.
  * How devices on the Internet are identified by their IP address.
  * How the Domain Name System translates names into numeric IP addresses.
  * How data is broken into packets and routed through the Internet from sender to destination.
  * The difference between the Transmission Control Protocol and the User Datagram Protocol.
  * How reliability and performance tradeoffs can be made by choosing TCP or UDP.
  * How to obtain information about your computer's NIC, IP address, DNS server, etc.
  * How to use ping, traceroute, ifconfig, and nslookup to diagnose network problems.

==== Preparation ====

This week's preparation is to watch some short videos about networks and the Internet.

=== Introduction ===

Begin by watching four short videos (averaging about 5.5 minutes each) on YouTube.
These videos present an easy introduction to the topic.
They also have very good captions in Japanese.
To turn the captions on, click the "Subtitles/Captions" button
at the bottom-right of the video window.
Next to that button is a "settings" button (it looks like a cog wheel)
where you can change the captions to your preferred language.

**1. What is the Internet?**  (3.5 minutes)

https://www.youtube.com/watch?v=Dxcc6ycZ73M&list=PLzdnOPI1iJNfMRZm5DDxco3UdsFegvuB7&index=1

**2. Wires, cables, and WiFi** (6.5 minutes)

https://www.youtube.com/watch?v=ZhEf7e4kopM&list=PLzdnOPI1iJNfMRZm5DDxco3UdsFegvuB7&index=2

**3. IP addresses and DNS** (6.5 minutes)

https://www.youtube.com/watch?v=5o8CwafCxnU&list=PLzdnOPI1iJNfMRZm5DDxco3UdsFegvuB7&index=3

**4. Packets, routing, and reliability** (6 minutes)

https://www.youtube.com/watch?v=AYdF7b3nMto

If you do not understand some word or phrase then pause the video and use
Google, Wikipedia, or even YouTube to find a brief explanation online.
If the explanation does not help (has too much or too little information,
or is badly written) move rapidly on to a different explanation.
If you cannot find any explanation that makes sense, ask on Teams for recommendations.

=== Technical details ===

Deepen your knowledge ideas by watching the following videos.
The total time is about 25 minutes.
Use the summaries below to make sure you understood the important ideas presented in each video.

**1. NICs** (3.5 minutes)

https://www.youtube.com/watch?v=oo-tn17rUBo

Network Interface Cards (NICs) connect computers, printers, and other devices to a network.
A single device can have multiple connections to one or more networks.
NICs can have connectors for electrical "copper" Ethernet wires (RJ45)
or light "optical fibre" (SFP, Small Form-factor Pluggable).
Every NIC has a unique, hard-coded (cannot be changed) Media Access Control address.
This address is used to identify pairs of NICs that are
communicating directly with each other on a local area network (LAN).

**2. Local area network devices** (7 minutes)

https://www.youtube.com/watch?v=1z0ULvg_pW8

A **hub** connects all devices on a network together.
A hub is not intelligent: it copies every packet received to all the other connected devices.
This creates unnecessary traffic and wastes bandwidth.

A **switch** is intelligent: it learns the physical (MAC) addresses of
all the devices connected to it.
A switch sends received packets only to the intended destination device.
Switches reduce unnecessary traffic on the network.
Hubs and switches form a local area network (LAN) and
communication is always direct between to devices based on MAC addresses.

A **router** communicates both with a LAN and with another external "wide area" network (WAN),
usually the Internet.
The Internet uses IP (Internet Protocol) addresses.
When a router receives a packet the IP address determines
if the packet is meant for the LAN or WAN.
If the packet is meant for the LAN it is delivered directly to a local device.
If the packet is meant for WAN it is forwarded directly to another router
connected to another LAN.
Routers are a kind of gateway for each LAN.
The Internet is made of many routers connected together.
Hubs and switches //create// networks, routers //connect// networks together
(as an **inter**connected **net**work = an **internet**).

**3. Breaking data into packets** (5 minutes)

https://www.youtube.com/watch?v=oj7A2YDgIWE

Networks are connected together to make the Internet.
Billions of devices are connected to the Internet.
To get data from one part of the world to another we need to
package the data and then send it through many routers to its destination.
Before it is transmitted over the network,
large data is chopped into smaller pieces called PACKETS.
Each is sent individually along with information about how to reconstruct the original data.
The destination of a packet is indicated by an IP address.
Each router understands how to forward a packet to the next router,
one step or "hop" closer to the final destination.
When the destination router is reached, the data is sent to
the local device that should receive it.
The device reassembles the original data which is then presented to the user.

**4. Naming and DNS: the Domain Name Service** (6 minutes)

https://www.youtube.com/watch?v=mpQZVYPuDGU

Computers are identified by numbers, but humans like to use names instead of numbers.
To make communication easier, the domain name system (DNS)
resolves (translates) domain names to numbers that are IP addresses.
You can use the Internet with IP addresses if you like, but using names is much easier.
DNS works like a telephone directory: before calling a remote telephone
you look up the number based on the name of the person you want to reach.
To turn a domain name into an IP address number,
your local "resolver" (usually your router) first asks a root server.
Your resolver is then sent to a series of other servers, each one closer to "yahoo.com".
Finally it asks the name server owned by Yahoo about "yahoo.com"
which provides an authoritative answer.

E.g., to turn "yahoo.com" into an IP address:

  * Your resolver asks a root server: "What is the address for 'yahoo.com'?"
  * The root server tells your resolver: "Ask the top-level domain (TLD) server in charge of '.com' addresses"
  * Your resolver asks the '.com' TLD server: "What is the address for 'yahoo.com'?"
  * The '.com' server tells your resolver: "Ask 'ns1.yahoo.com', the authoritative server in charge of all the 'yahoo.com' addresses"
  * Your resolver asks the 'ns1.yahoo.com' server: "What is the address for 'yahoo.com'?"
  * The 'ns1.yahoo.com' server tells your resolver: "74.6.231.20" (the actual answer to the original question)
  * Your resolver then remembers (caches) the answer so that the next time you need to look it up, the answer is immediately available.

**5. Communication protocols: TCP vs UDP** (4 minutes)

https://www.youtube.com/watch?v=uwoD5YsGACg

TCP (transmission control protocol) is one of the main protocols used on the Internet.
TCP guarantees that all transmitted data is received, in the correct order.
First a connection is established between two computers that want to communicate.
This "synchronises" the communication (using "SYN" packets)
so the computers know what packets have been sent and received.
Data is then transferred between the computers.
The receiver tells the sender about missing packets, which are re-transmitted.
The receiver sorts received packets into their original order,
reassembles the data inside them, and delivers it to the local application.

Typical uses of TCP:
  * e-mail,
  * video/audio streaming,
  * other information that must not have gaps or be rearranged.

UDP is a connectionless protocol.
There is no initial connection, no synchronisation between sender and receiver,
and therefore no guarantee of reliability.
UDP does not care (or even know) if data is lost or
if it arrives out of order at the destination.
Because of lower overheads (no synchronisation, retransmission, reordering)
UDP is faster than TCP.

Typical uses of UDP:
  * real-time sensor data,
  * character position updates in online video games,
  * other information that is constantly and rapidly renewed.

**6. Networking tools** (4.5 minutes)

https://www.youtube.com/watch?v=vJV-GBZ6PeM

The **ping** command sends packets from one computer to another
requesting a simple "I am here" reply.
Use it to detect router errors ("destination host unreachable"), DNS naming errors, etc.

The **traceroute** (tracert on Windows) shows the route a packet takes
through the Internet to reach another machine.
Use it to pinpoint where a problem lies if packets do not reach their destination.

=== Background information ===

Watch these videos if you would like additional information.

**1. How does the Internet work? Networks and addresses explained** (10 minutes)

An alternative explanation of how the Internet works:
https://www.youtube.com/watch?v=82m2du-zgmY

**2. The Internet vs. The Web** (5 minutes)

A brief history of why and how the Internet was created:
https://www.youtube.com/watch?v=CX_HyY3kbZw

**3. Network troubleshooting using PING, TRACERT, IPCONFIG, and NSLOOKUP** (14.5 minutes)

More networking tools (ifconfig, nslookup):
https://www.youtube.com/watch?v=AimCNTzDlVo

Note: The "traceroute" and "ifconfig" commands are called "tracert" and "ipconfig" on Windows.
If you have MacOS or Linux, "traceroute" and "ifconfig" work
similarly to the commands described in this video.


</WRAP> /* syllabus */

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