Internet

• Global network of billions of electronic devices which acts as servers or clients.
• Servers are machines which provide services to other machines.
• Clients are machines used to connect to those services.

Q. How do we distinguish each machines? Q. How do we transmit data

WAN (Wide Area Network):	LAN (Local Area Network)
Group of computers that span large geographic area.	Group of computers connected in limited geographic area.
Eg: Internet	Eg. Home Network, School Network

What is IP Address?

• An IP address is a unique address that is used to identify computers or nodes on the internet.
• Addresses are in this form: #.#.#.#

[!info] Your router is the only device on the internet with this public IP Address - Inside your home, your devices have most likely received a local IP Address from router. - This local IP Address is unique on your network however, there might be other devices with same IP Address on other networks (LANs)

• IPv4: Numbers range from 0 to 255. IP addresses are 32-bits, meaning that these addresses could accommodate over 4 billion addresses.
• IPv6: Newer versions of IP addresses can accommodate far more computers! IPv6 #### What is Subnet mask? 255.255.255.0 Here 255 means that IP address amongst the devices in LAN are common where as 0 means unique to devices. #### What is Default Gateway? It is the Local IP Address for the router/modem that you are connected to.

graph TB
  A["Your friend. In another house! 192.168.1.5"] --> B[["Router 192.168.1.1 (Local IP)\n Router 77.45.82.17 (PUBLIC IP)"]]
  K["Your friend... In another house! 192.168.1.4"] --> B 
  B <--> D["The Internet"]
  D <--> E[["Router 77..45..19..6 (PUBLIC IP)\nRouter 192..168..1..1 (LOCAL IP)"]]
  E --> G["Computer 1 192.168.1.4\n Our machine running our server on port 12345!"]
  E --> I["Computer 2 192.168.1.7"]
  E --> J["Computer 3 192.168.14"]
  E --> L["Computer S 192.168.4.1"]
  E --> M["Computer A 192.168.2.1"]

Port Address

Port Address is the address that a specific application or service runs on the given computer. Port addresses can range from 0 to 65535. Some of them might be reserved for well know protocols and services. #### Common Ports and Services Port 21 - FTP File Transfer Protocol Port 22 - SSH Secure Shell Port 25 - SMTP Simple Mail Transfer Protocol Port 80 - HTTP Hyper Text Transfer Protocol Port 443 - HTTPS Hyper Text Transfer Protocol Secure

• netstat -na gives entire list of open ports on Windows

[!Warning] Port Address conflicts [todo]

Internet Protocols

Transmission Control Protocol

• TCP is a connection based protocol.
• Connection must be made before any data is sent.
• Used to establish a reliable, error free connection. It ensures all data was sent is received and it keeps all sent packets organized.
• For Example HTTP and HTTPS service is based on TCP Protocol

User Datagram Protocol

• UDP is a connectionless protocol.
• No connection has to be made before sending data
• UDP tends to be faster but you may not receive the data in order and you could have missing data.

What is a Socket?

• A socket is one endpoint of a two-way communication channel between two programs running on a network.
• A socket is a way of connecting two programs over a network so that they can exchange data.
• When we create a socket we must specify the internet protocol and the communication protocol the socket will use.

—

What is HTTP (Hyper Text Transfer Protocol)?

• It is used to structure request and responses over the internet to transfer data (html, images, css, etc) from one point to another.
• Transmission is based on TCP Protocol. TCP Manages the channels between browser and server. TCP is used to manage many different types of internet connection between devices.
• HTTP is the command language that devices on both side of the connection must follow to communicate. ## Basic Working

1. When webpage is requested using a URL. then browser opens a TCP Channel to the server that responds to that URL.
2. Once the TCP Connection is established, the client sends HTTP GET request to retrieve the webpage it should display.
3. After that server sends the response and closes the TCP Connection. >This process is repeated each time the browser is opened or new URL connection needs to be established,

In Depth Working GET Request

1. URL Requested
2. Browser extracts the protocol http or https and uses the specified.
3. Identifies the domain name from the URL and queries the Internet Domain Name Server (DNS/Domain Name System) to return Internet Protocol Address.
4. Browser, the client opens the opens a connection using the http protocol to the server at the IP Address resolved above.
5. Client initiates a GET Request

-- CLIENT GET REQUEST --
GET / HTTP/1.1
Host: www.codecademy.com

HTTP vs REST (related but distinct concepts)

HTTP (Hypertext Transfer Protocol): - HTTP is the foundation of any data exchange on the Web. - It’s a protocol used for transferring hypertext (text that includes links to other texts, like web pages) over the internet. - It defines how messages are formatted and transmitted and what actions web servers and browsers should take in response to various commands. - In simpler terms, it’s the set of rules that allows your web browser to request a webpage from a server, and the server to send that webpage back to your browser as response.

REST (Representational State Transfer): #Guidelines https://www.codecademy.com/article/what-is-rest - REST is an architectural style, a set of principles that are often applied when building web services. - It’s not a protocol like HTTP; instead, it’s a set of guidelines for creating scalable and stateless web services. - RESTful services use standard HTTP methods (like GET, POST, PUT, DELETE) for operations on resources (like data entities), and they represent these resources as URIs (Uniform Resource Identifiers). - RESTful services typically use JSON or XML for data exchange. - In simpler terms, REST is an approach to designing networked applications in a way that is scalable, stateless, and relies on standard HTTP methods.

[!Info] Stateless Web Service: Each request from client is independent & carries all information. So, the server doesn’t store the client’s state between requests and treats each requests as isolated transaction.