Question 1

I thought IP address was specific for a device. Why do websites have an IP address ? I'm confused now.

Accepted Answer

MAC addresses are specific to devices. Typically a network interface device will have a sticker or label with its MAC address on it.

IP addresses are not specific to devices. Often, when you hook up a device to a network it requests an IP address to be assigned to it. The amount of time it is allowed to keep that IP address can vary (some networks may even choose to permanently assign a device an  IP address ). It is even possible, through the use of Network Address Translation, for many devices to share the same IP address.

So why don't we just use MAC addresses for websites ?
A MAC address is just for a single network interface device. If that device broke down and had to be replaced or the website want to use several different devices to balance the load of requests, a single MAC address wouldn't work.

Hope this makes sense

Question 2

How do you know if you are using a fake website and not the real one?

Accepted Answer

If you are using https: then your browser will receive a certificate signed by a certification authority confirming that the website is who it says it is.

For example on Chrome, if I click on the lock by the web address and the tab for "Connection" it will have a link to the "Certificate Information".

If you are using just plain http: then you can't be certain whether the site is fake or not.

Question 3

How can you figure out what your computer's address is?

Accepted Answer

For Windows command prompt: use the command `ipconfig`
For Unix command prompt: use the command `ifconfig`

It will look something like this: X.X.X.X where X are numbers

If the IP address you find looks like 192.168.X.X (it starts with 192.168), then:
- You only found out your local IP address on your internal network. 
- Your router is using NAT (network address translation), so that outside world only sees one IP address, but all your devices on your local network can use it
  (For more info see: https://en.wikipedia.org/wiki/Network_address_translation) 
- To find the IP address that the outside world sees use https://www.whatismyip.com/
  (alternatively you can run the administrator tools for you router and it should tell you)

Question 4

if my hardware (say PC) is connected to a LAN will it have same IP address for local network and public network (internet) ?

Accepted Answer

It depends.

If you are not using Network address translation (NAT) then your external IP address will be the same as your local IP address.

If you are using NAT (which most home networks with a router will be using) your router will present a single external IP address to the world, and your devices on your local network will have addresses between 192.168.0.0 and 192.168.255.255. NAT works by making up port numbers for each device on your local network. When one of your devices sends a packet to the internet, NAT will replace the source IP address with the external IP, and the source port with the port number it made up for that device. When NAT receives a packet, it looks at the port number for the packet and then forwards it on to the device associated with that port number.

Why do we use NAT ? Under the dominant standard IPv4 there are a limited number of IP addresses, and buying them is expensive. NAT lets many devices share one IP address. Under IPv6 (the newer standard, which is slowly spreading) there are so many IP addresses that every device can have its own IP address, which will make NAT, for the most part, unnecessary.

For more info see: https://en.wikipedia.org/wiki/Network_address_translation

Question 5

can someone comment on the concept of a logical port number, explanations I could find are way too abstract for me to comprehend. thank you.

Accepted Answer

Suppose I wanted to send a package to Bob Smith at the XYZ company.
I would have to write the address of the XYZ company on the package, and I would also have to write the name of Bob Smith on the package. The delivery person would deliver the package to the mail room at the XYZ company. The mail room at XYZ would then sort out the mail and deliver Bob Smith his package.

A similar thing happens with packets on the internet. The IP address is the address, that tells the internet where the packet needs to go to on the network. The port number tells the computer where the data needs to go once it has reached the computer e.g. one packet could be part of a picture loading on your web browser, and another packet could be part of an update for some program.

Question 6

I have several questions in regard to IP addresses:)
1) Do Internet Service Providers and websites buy IP addresses? 
2) When my computer connects to different networks it uses different IP addresses?
3) If some IP addresses are temporary, how did people catch hackers based off that?
4) If a hacker knows the IP address I'm using (which is temporary) and tries to hack the address when someone else uses it, will the hacker be hacking someone else's computer?
5) How can I protect my IP address from hackers?
6) Does Cloud mean uploading local files to servers so that given the IP address of the server we can use different IP addresses to access the files?
7) How does the DNS servers know the IP addresses given the DNS of websites at the very beginning? How do the website set up the "memory"?
Thanks in advance for your generous answers! Love the video - always wanted to learn about how Internet works!!

Accepted Answer

1) Yes

2) For your computer to talk on a network it will be need an IP address. Typically, it will be leased an IP address by the local DHCP server. So, if you hook your computer up to your ISP's network, the ISP's DHCP server will lease you an IP address. If you take your laptop to school and hook up to their network, then the school network's DHCP serve will assign you an IP address. When the time expires, the DHCP server will lease you a different IP address.

3) IP addresses do change often. However, an ISP would own a block of IP addresses. It is possible that they might maintain logs of who was leased what IP address for how long. So, it might be possible to associate an IP address with a person. However, most hackers will hide their IP address behind proxy server(s), which can makes finding the hackers real IP address effectively impossible.( Don't believe what you watch on detective shows )

4) Yes

5) Usually it's not necessary to. If you really want to hide your IP address then hide it behind a proxy server. Alternatively, you could periodically tell your network interface device to release the old IP address and obtain a new one from the DHCP server.

6) The cloud is basically accessing servers by the internet. You could store files on them, run programs on them, etc. (depending on what the owner of the server has agreed to).

7) Typically when the website is registered with a domain name registrar e.g. Go Daddy, will add the entries to the authoritative name server. When other DNS servers look for that website, they will eventually ask that server.

Question 7

So an IP Address is 4 bytes?

That also means the highest CURRENT possible IP could be 1111111111111 or 255.255.255.255?

This also means the future IP will be 16 bytes and the IP will be in hexadecimal, right?

Accepted Answer

IPv4, the older standard, uses a 4 byte address 
One of the standard ways to present an IPv4 address is dotted decimal notation.
(Decimal values for each byte are seperate by '.', e.g. 192.168.1.1 )

IPv6, the newer standard, uses a 16 byte address.
(don't ask about where IPv5 went, it's a long story...) 
One of the standard ways to present an IPv6 address is 8 groups of 4 hexadecimal digits separated by colons e.g. 2001 : 0db8 : 85a3 : 0000 : 0000 : 8a2e : 0370 : 7334 (which can be further simplified to 2001 : db8 : 85a3 :: 8a2e : 370 : 7334 by eliminating leading zeroes and replacing consecutive blocks of zeros with :: )

Both of these protocols are in use, but the majority (>80%) of the traffic currently uses IPv4.

Of course, the IP addresses are just numbers, so there are many other ways to represent them.

By the way, 255.255.255.255 is reserved as a broadcast address in IPv4 (https://en.wikipedia.org/wiki/Broadcast_address)

Question 8

Is IP address for various devices or for websites only or for different different for both?

Accepted Answer

Anything that connects to the internet has to have an IP address. That means servers (which serve websites among other things), smart devices (like an iPhone or smart watch), and even cars.
An IP address is somewhat somewhat like a real address; if your smart watch asks a server for Khan Academy then the server has to know where to send the information once it finds it.

Question 9

How do you know if a website is fake? 6:09

Accepted Answer

The lock and the URL(Uniform Resource Locator) are your friends if a website has a HTTP(Hypertext Transfer Protocol) instead of the normal HTTPS(Hypertext Transfer Protocol Secure)  then you can easily be hacked and the URL like if I wanted to publish a replica of Khan Academy it would probably be something like this:
`kahnacademy.org`  This can make a huge difference because the website `khanacademy.org` is not  `kahnacademy.org`. Another tip is if there is a green lock on the side of the URL bar then that website is secured otherwise it is most likely a phishing website.

Question 10

When I accidentally type .khanacademy.com, how does it turn into khanacademy.org?

Accepted Answer

Here' some ways you could do that:
1) DNS:
In the DNS table have an entry that say that the canonical name for http://www.khanacademy.com is http://www.khanacademy.org
That way when your browser is looking up the IP address for .com it sees that it should look for the canonical name .org instead.

2) meta refresh tag
Add this code to the html of http://www.khanacademy.com:
```
<meta http-equiv="refresh" content="0; url=https://www.khanacademy.org/">
```
This would tell your browser to refresh after 0 seconds i.e. immediately, and redirect to http://www.khanacademy.org

3) HTTP 301 redirect code
When the browser requests http://www.khanacademy.com the web server could respond to the browser with
```
HTTP/1.1 301 Moved Permanently
Location: https://www.khanacademy.org/
```
Your browser then looks up http://www.khanacademy.org

4) JavaScript
If JavaScript is enabled one could include and run the following code on the http://www.khanacademy.com webpage:
```
window.location = "https://www.khanacademy.org/";
```
This will redirect the browser to load https://www.khanacademy.org/

Which way does Khan academy use ?
A curl request for http://www.khanacademy.com responds with:
```
HTTP/1.1 301 Moved Permanently
Cache-Control: max-age=900
Content-Type: text/html
Location: http://www.khanacademy.org
Server: Microsoft-IIS/7.5
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: (The date and time I made the request)
Content-Length: 0
Age: 1
Connection: keep-alive
```
so we can see that they are using a HTTP 301 redirect code

Hope this makes sense

Course: Code.org > Unit 1

IP addresses and DNS

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Video transcript