Overview

Teaching: 15 min
Exercises: 15 min

Questions

• What is a LAN?

• What are ports and what do they do?

• What is a domain name?

• What is a hostname?

Objectives

• Know your computer’s IP address and if it is a LAN address.

To put cloud computing into perspective, lets first take a look at how your personal computer interacts with the Internet. A first model of what the internet looks like might be something like a bunch of computers connected together with the Internet being the glue that connects them together into a computer network allowing them to share resources and exchange data.

To improve our model of the Internet lets add a little more detail to this picture.

Local Area Networks

Many of us have a local network at home and at work. We have a router (often wi-fi) which our computers connect to which routes information from one computer to another. We can communicate between software programs and transfer files between computers on such a network even if they were not connected to the Internet. This is what many of the first networks of computers were like, though they used wires instead of wifi. These networks only allowed sharing of information between local computers, often called a Local Area Network (LAN).

Within a LAN or local area network, IP addresses often have the form 192.168.###.### by convention. If you see an IP that looks like this it is likely the IP address of a computer on a local private network such as your home or office network.

Layers of Networks

Many of these local area networks exist around the world and the Internet connects these smaller networks together which is where the name Internet comes from, interconnected network (Internet).

To connect your LAN to the rest of the Internet your router is connected to an Internet Service Provider (ISP). Your computer then accesses the Internet through the router often also called a “gateway”. How does a computer on your network know how to exchange information with a computer on your Neighbour’s LAN? Again IP addresses are used. Each LAN connected to the Internet gets a unique IP address. This is sometimes referred to as a “public IP” as it can be viewed by anyone connected to the Internet. This is in contrast to IPs on a LAN which are only meaningful within the LAN.

How many unique IPs are there?

IP address are compose of numbers separated by . Each number is chosen from the range from 0 to 255, or one of 256 possible numbers. An IP is made up of 4 such numbers separated by .s so all possible combinations of these numbers gives 256⁴=4.294e9 (about 4 billion) possible IP addresses. There are now ~7 billion people in the world and in 2011 we ran out of IPv4 address (e.g. IP address of the form ###.###.###.###). This is why IPv6 address were introduced which provide approximately 3.403e38 unique addresses. However IPv4 addresses are still widely used and very common.

Ports

If your neighbour wanted to share some information with you on a website located on his device with local address 192.168.1.103 how could your computer know to connect to that device to get that information? If you know the IP address of your neighbour’s router (206.113.222.122) you could uniquely identify your neighbours LAN, but how to route the request for their website to the local device? Your neighbour must configure their router to forward traffic (data being sent back and forth) to the device with local address 192.168.103, but which traffic? What if your neighbour wanted some information to be routed to one device and other information to be routed to another? The solution to this issue is to use ports. Different types of information requests are associated with different ports and the local network router can be configured to forward a request associated with certain port to a specific device (known as port forwarding). In the case of a website the convention is to use port 443 or 80 depending on weather the data sent and received is encrypted (443) or not (80). So your neighbour would setup a rule on their router to forward traffic arriving on port 80 and or 443 to the server running the website at the local address 192.168.1.103.

You can combine IP address and ports with a : for example 206.113.222.122:80 could be entered into your web-browser to access a website on your neighbours device with local IP 192.168.1.103 once your neighbour configured their router to forward requests on port 80 to that device.

How does this relate to cloud?

When we start using the cloud you will have a virtual local area network, or private network, to which your virtual computers will connect. This virtual local area network or private network will act very similarly to the LAN described above. You will have private IPs in the range 192.168.###.### and you will be able to manage which ports are accessible from the internet to your virtual computers.

Domain Names

All the services you access over the Internet work by using IP address and ports to access another computer connected to the Internet which provides your computer with information (text, audio, video) stored on another computer.

But when you want to go to a website you don’t usually enter an IP address usually something like google.ca. This is what is known as a domain name. Domain names can be purchased from a domain name registrar (e.g. godaddy.com, rebel.ca). The domain name can then be linked to an IP address and requests to a domain name are translated to requests to an IP address. Domain name registrars run servers (special purpose computers) which do these domain name to IP address translations which are known as Domain Name Servers (DNS).

The .ca part of google.ca domain name is known as the top level domain name. Within a given domain you can have many sub-domains for example google is a sub-domain of the ca domain and translate is sub-domain of the google.ca domain. Each additional sub domain given to the left of the top level domain name can be referred to by the depth they are to the left, for example google is a second level domain name, and translate is a third level domain name.

A domain name does not have to point to a single device, but may also point to a network with many devices connected to it.

Hostnames

A hostname identifies a specific device on a network. For example you could have a computer with a hostname bob and other computers on that network could refer to that device simply as bob. However, outside that network you would have to include the domain name for the network that the computer was on, for example bob.example.ca if example.ca pointed to the network that the device bob was on. In this case bob.example.ca is referred to as a Fully Qualified Domain Name (FQDN) because the domain name hierarchy can be followed up to a top level domain name (in this case ca).

What is your computer’s IP?

Go to ipv4.icanhazip.com and see what your IP address is. To see a full description of how to use this website go to major.io/icanhaxip-com-faq.

Next try determining your IP from the command line.

If using Windows:

1. open command prompt by searching “cmd.exe” in the start menu
2. type ipconfig

If using Linux or Mac:

1. open the terminal (try searching for terminal)
2. type ifconfig

Depending on the network configuration you may or may not see the same IP as listed at ipv4.icanhazip.com why or why not?

Solution

• If you see a different IPs, does the IP you see with either ifconfig or ipconfig look something like 192.168.XXX.YYY. If so, you are likely on a local network and connecting the the Internet through a gateway, such as a router. This is likely what you would see at home if you have a router connected to your modem.
• If you see the same IP it likely means you are either directly connect to your ISP modem, or your institution provides unique public IPs for devices connected on their network. You likely just see a local or LAN IP address on the command line, while from ipv4.icanhazip.com you will see the IP address given to the router you are connecting to the Internet through.

What happens if I specify the wrong port?

Did you know that you can specify a port to use when entering a URL into your browser? You can actually do this by appending a URL with a : and the port number. Lets try it.

1. Enter a URL into your browser of a known website (e.g. http://google.ca). Do not yet go the site.
2. Then append :80 to the end and press return

Do you get the site you expected? What happens if you try port 81, or 22, or 443 the port for ssh connections? Port 80 is used for the HTTP protocol. Most web-servers will be listening on port 80 for incoming requests.

Solution

If you enter the wrong port your browser will likely spend some time trying to connect to the server on that port. If you wait long enough you will eventually see a message like:

This site can’t be reached

google.ca took too long to respond.
Search Google for google
ERR_CONNECTION_TIMED_OUT

What is google.ca’s IP address?

1. Go to ip-lookup.net
2. Select “Domain Lookup”
3. Enter a domain name into the search box (e.g. google.ca)
4. Copy and paste IP address into your browser

Do you get to the same page that you would have using the domain name?

What is the top level domain name?

What is the top level domain in ipv4.icanhazip.com

1. ipv4.icanhazip.com
2. ipv4
3. com

Solution

1. This is the full domain name and not just the top level domain.
2. ipv4 is the lowest level domain and most specific component of the domain name.
3. Correct, com is the top level domain having many subdomains.

Key Points

• Computers often have an IP on a LAN and connect to the Internet through a router.

• You can see what your computer’s IP address is by using either ipconfig on windows or ifconfig commands on linux or Mac.

• You can see your computer’s public IP using the site ipv4.icanhazip.com.

• Ports allow computers and routers to differentiate types network traffic.