Advantages of Using Networks
Network Classifications
Ethernet & Network Layers
Network Protocols
Good notes will help you organise and process data and information
Imagine a gigantic worldwide Ethernet network of interconnected switches. For each switch to be able to switch incoming data frames to the correct port, the switch would need to have some sort of list where it can look up the physical address of every connected device in the network.
This list would be huge, and it would take a lot of memory storage and processor time to use it and keep it updated. And how would the system cope if a device moved? What about technologies that don’t use Ethernet addresses? It just wouldn’t work!
This is why a new addressing scheme was invented: the IP address system. Like postal addresses, IP addresses are logical addresses: they have a logical structure and provide a set of instructions for reaching a destination.
Take this postal address as an example:
It can be read from the bottom up, so a letter with this address would be first sent to the UK, then to Birmingham, and then to the right street and house number.
When IP addresses were invented, in the 1970s, it was decided to make them 32 bits long. 32 bits allow for 4 billion unique addresses, enough so that each person on Earth at the time could have one. We didn’t know how big the interconnected network — or internet — we were building would become!
This 32-bit IP address is usually not written in binary. Instead, it is split into four 8-bit chunks (bytes), and each chunk is converted to decimal. The address is then written in dotted decimal notation, with the four decimal values separated by dots.
So the IP address 11000000101010000000111000010111 becomes:
11000000 10101000 00001110 00010111 = 192.168.14.23
Mobile devices (including laptops) are automatically given a different IP address each time they join a different network, because the network bits of their address need to identify the new network. Later in the course, you will learn how IP addresses are automatically allocated.
Sending data using IP addresses is very similar to sending data using MAC addresses and Ethernet frames:
The source and destination IP addresses are sent first (together with some additional fields you will learn about later). Then follows the actual data to be transmitted. This structure is called an IP packet, and it looks different to an Ethernet frame.
HTTP is the protocol most internet users have heard of, since it’s a common prefix to the URL you see in a web browser’s address field.
HTTP requests web pages from a web server and can send information when a user fills in a form. The web pages that get sent back are formatted using Hypertext Markup Language (HTML).
HTTP is gradually being replaced with Hypertext Transfer Protocol Secure (HTTPS). HTTPS encrypts the traffic between the application and the server to protect user’s data. Many browsers denote HTTPS with a padlock symbol to show the link is secure. You should only enter personal information into websites that support HTTPS.
As the name suggests, FTP moves files from a client device to a file server. The FTP application may be a dedicated application on the client device or built into a web browser or other software.
Some examples where FTP would be used are:
Generally, if you end up with an actual file in a folder on your device, you probably used FTP to get that file.
Email was initially one of the main uses of the internet and still accounts for a significant number of data transactions.
Users have email addresses such as jsmith@raspberrypi.org. A specific email server manages all the accounts with the same address after the @.
When a user sends a message, using an email client, an app, or a webmail page, the message sent to the mail server using a protocol called SMTP.
Once an email arrives at the destination mail server, it is stored in the recipient’s ‘mail box’ until they connect to read it.
If a user connected using the Post Office Protocol, or POP, then the actual email is transferred to the client’s device and deleted from the server.
This used to cause problems when groups of people were sharing an email account, e.g. a specific company account that people message when they need support (such as support@abc.com).
To combat this and give additional functionality, the Internet Message Access Protocol, or IMAP was developed. IMAP allows messages to be retrieved while a copy is maintained on the server; a specific request to delete an email is sent when the email is no longer needed.
The structure of the da
A set of conventions and rules that govern the communications between devices is called a protocol.
The 1500 byte limit, the CSMA/CD rules, and the need to send destination and recipient addresses are all part of the Ethernet protocol.
The four non-data fields together ‘wrap around’ the data as the Ethernet Frame, in what is called encapsulation.
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