What is a network?

• Computers that are connected together

What is Internet?

• The collection of these computer networks

Protocols:

• These are just rules that are defined by the internet society

• TCP:

  • It ensures that the data will reach its destination and it will not be corrupted on the way.

• UDP:

  • There would be chances of data corruptibility when we use this protocol.

• HTTP:

  • This is used by the web browser.
  • It defines the format of the data that is being transferred between web clients and servers

• After getting a response from the internet, now the modem will decide which device initially sent the request -- this is done using NAT.

• IP address : decides to which device to send the data
• Ports : decides to which application to send the data in that device
• Ports are 16-bit number
  • Tota port nos. possible->2^16

Good To Know Stuff:

• All the HTTP stuff will happen on port 80.
• Reserved ports--> 0-1023
• Registered for applications(e.g. MySQL,mongoDb)--> 1024-49152

Some Terminologies:

• LAN:

  • If we have some devices connected in a small house/office then its known as LAN
    • e.g.- Ethernet cable, WIFI

• MAN:

  • If the connection is across a city

• WAN:

  • If Connection is countries
    • e.g.- Optical fibre cable

• SONET:(Synchronous Optical Network)

  • It carries the data using optical fibre cable, hence it can cover larger distances

• Frame Relay:

  • It is a way for you to connect LAN to the internet

What is Modem?

• It is used to convert the digital signal into analogue signal and vice-versa

What is Router?

• It is a device that routes the data packets based on their IP Addresses.

Topologies:

• Bus:

  • 
  • Disadvantages:-
    • If the central wire gets broken,entire system is spoiled
    • Only one person can send data at one particular time.(Through central wire)

• Ring:

  • 
  • Every system communicates with one another
  • Disadvantages:-
    • If one of the cables break,then its spoiled
    • Lot of unnecessary calls are being made

• Star:

  • 
  • Disadvantages:-
    • If Central device fails then system is spoiled

• Tree:(Bus+star)

  • 

• Mesh:

  • 
  • Every single component connected to every single component
  • Disadvantages:-
    • Expensive
    • Scalability issues

Structure Of Network:

• OSI Model:(Open System Interconnection Model)

  • 7 layers of OSI model:
    • Application layer
    • Presentation layer
    • Session layer
    • Transport layer
    • Network layer
    • Data link layer
    • Physical layer

  • APPLICATION LAYER:

    • Human-computer interaction layer

  • PRESENTATION LAYER:

    • The presentation layer will convert the data into machine representable binary format -- This process is known as translation
    • This layer also provides abstraction.

  • SESSION LAYER:

    • This layer helps in setting up and managing the connections and it enables the sending and receiving of data followed by termination of connected sessions.

  • TRANSPORT LAYER:

    • This layer works with data and makes sure it is transported easily.
    • It does it in 3 ways:
      • Segmention: The data which is received from session layer will be divided into small data units called Segments.
        • Segments <- Every segment will contain the source and the destination's port no. and a sequence no.
          • sequence no. helps to reassemble the segments in correct order
      • Flow control: It controls the amount of data that's being transported
      • Error control: It adds something known as "checksum" to every data segment,that way it can figure out whether data that was received was good or bad

  • NETWORK LAYER:

    • This layer basically works for the transmission of received data segments one from computer to another that is located in different networks.
      • e.g. Routers
    • Function of network layer:
      • Logical addressing:
        • Network layers assign the sender's and receiver's IP addresses to every segment and it forms an IP pocket. So that every data packet can reach the correct destination.

  • DATA LINK LAYER:

    • This layer allows you to directly communicate with the computers.
    • Data Link layer will receive the data packet from the network layer and this data packet contains the IP addresses of both sender and receiver
    • Note:
      • Logical addressing: This is done at the network layer which is IP addresses and stuff.
      • MAC addressing: This is physical addressing, the address of sender and receiver are assigned to the data packet to form a frame
        • Frame: It is a data unit of the data link layer.
      • Remember: MAC address is a 12 digit alpha-numeric no. of the network interface of the computer( MAC-> Media Access Control)

  • PHYSICAL LAYER:

    • This is the hardware part
    • In this layer you work with cables and stuff
    • This layer will convert the data into electrical/optical signal
  • 

• TCP/IP Model: (Practically used model)

  • It is known as Internet Protocol Suit
  • 5 layers of TCP/IP Model:
    • Application layer
    • Transport layer
    • Network layer
    • Data link layer
    • Physical layer
  • Deep Dive on TCP/IP Model

    • APPLICATION LAYER :

      • It is the main layer where the user interacts, it consists of applications
        • e.g. Whatsapp, browser etc.
      • It is present in individual devices.
      • Client-Server Architecture
        • 
        • What is a server?
          • It is a system that controls the website you are hosting.
        • Data centres:
          • Collection of many servers at a place
          • They may have static IP addresses
          • High upload speed
        • What is a ping?
          • It measures the round trip time for messages sent from the originating host to destination
      • P2P architecture
        • 
        • Various devices get connected with each other
        • There is no one server present
        • Each computer acts both as server and client
          • e.g. Bit torrent
        • SOME TERMINOLOGIES:
          • Repeater-
            • Operates at the physical layer
            • Its job is to regenerate the signal over same network before it becomes too weak or corrupted
            • Its a two port device
          • Hub-
            • It is a multiple port repeater
            • It connects multiple wires from different branches
            • It regenerates the network
          • Bridge-
            • Operates at the data link layer
            • Bridge is a type of repeater
            • It has an additional functionality -- It can filter out the content by reading MAC address of source and destination
          • Switch-
            • It is a multi port bridge that can boost the performance and efficiency
            • It can perform error checking before forwarding the data,so it makes it very efficient
          • Gateway-
            • It's a passage to connect two networks together that may work upon different networking models
      • Some Gyaan On Protocols:
        • HTTP:(Hyper-Text Transfer Protocol)
          • This is used by the web browser.
          • It defines the format of the data that is being transferred between web clients and servers
        • DHCP:(Dynamic Host Control Protocol)
          • It allocates IP address to the devices connected to the network
        • FTP:(File Transfer Protocol)
          • Files are transferred using this protocol
            • Note: It is not used these days anymore
        • SMTP:(Simple Mail Transfer Protocol)
          • It is used to send emails
        • IMAP & POP3:
          • These are used to receive emails
        • SSH:
          • If you want to login to a terminal of someone else's computer then use SSH
        • VNC:(Virtual Network Computing)
          • Its used for graphical controlling
        • Telnet:
          • It is basically a terminal emulation that enables the user to connect to a remote host/device using a telnet client
        • UDP:
          • Stateless Connection
          • Data may be lost
          • This protocol is used in video calling
      • Program:
        • Program is just like an app
          • For e.g. Flipkart,amazon etc
        • Process:
          • One of the features of the program
          • One program can have many many processes running at the same time
        • Thread:
          • It is lighter version of process.
          • One process can have multiple running threads
        • Remember: Thread is doing one single job
      • Sockets:
        • When you need to send messages from one system to another system, use sockets for that
      • Ports:
        • Remember:
          • IP address tells us which device we are working with.
          • Ports tells us which application we are working with.
        • Ephemeral ports:
          • Let's say you have port 80 reserved for HTTP, so the application will internally assign itself random port no., when the process is done it will be freed.
          • Ephemeral ports can exist on the client-side but on the server-side you have to know the port no. .
      • HTTP:
        • It's a client-server protocol and it tells us how the server will send back the data to the client
        • It's a stateless protocol which means that the server will not store any information about the client by default
        • HTTP uses TCP protocol
        • TCP makes sure that all the data is received and everything
      • HTTP Methods:
        • Methods tells the server what to do and all
        • 1. GET: You are requesting some data
             • (Give me this webpage, this youtube video etc)*
        • 1. POST: I am a client and I am giving something to the server
             • (e.g.-> Webform i.e Username,password etc)*
        • 1. PUT: It put data at a specific location
        • 1. DELETE: If you want to delete data from the server. You will have to send a delete request
      • STATUS CODES:
        • 1XX -> Informational
        • 2XX -> Success
        • 3XX -> Redirecting
        • 4XX -> Client error
        • 5XX -> Server error
      • Cookies:
        • It is a unique string
        • It is stored on the client's browser
        • When you visit a website for the first time--> it will set a cookie and after that whenever you make a new request, in that request's header cookies will be sent.The server will know this request is coming from Amaan's computer
        • Remember: Cookies have an expiration date
        • Third Party Cookies:
          • These are cookies that are set for the URLs that you do not visit
      • Emails:
        • How do Emails work?
          • SMTP(Simple Mail Transfer Protocol) is used to send an email
          • POP3 & IMAP are used to receive email
          • Above protocols are application layer protocols
          • POP:
            • You first connect to the POP server (using TCP) , it does all the authorisation and stuffs and then you(client) asks the server for all emails
            • 
          • IMAP : (Internet Message Access Protocol)
            • it allows you to view your emails on multiple devices
      • DNS:(Domain Name System)
        • We can say DNS is a directory
        • We know that Domain Names are mapped to IP addresses and we use a service to look at these databases
        • The most popular service for this is DNS
        • Mechanism: When you enter "Google.com",it will use DNS to find the IP address of google's server
          • So basically what happens is ->
            • You type "Google.com" --> HTTP protocol will take that domain name & use DNS and it will convert the URL into the address --> then it will connect to the server.
        • Sub-domain : Second-level Domain : Top-level domain
          • 
          • So instead of storing everything in one database, there are multiple databases for these 3 categories

    • TRANSPORTATION LAYER:

      • Transport layer is the layer that lies inside the device
      • Role of transport layer:
        • It's to take the info (Whatever message your friend is sending) from the network to the application within the computer
      • Note: The transportation of info from one computer to another computer is done by the network layer
      • Real Life Example of Network layer and Transport layer:
        • 
      • Protocols in transport layer:
        • TCP=> Data is not lost while using this protocol
        • UDP=> Data can be lost in UDP
      • Multiplexing & Demultiplxing
        • Multiplexing allows us to send all the messages, files etc to a lot of destinations via just one medium
        • Demultiplexing is the opposite of multiplexing
        • 
          • Note: -
            • Transport layer also takes care of congestion(traffic)
            • Congestion control algorithms are built in the TCP protocol
      • Checksums:
        • Imagine it like some number
        • You are sending data to your friend ---> Using that data you will calculate a value (i.e checksum) ---> Then your friend receives this modified data ---> Now the value of checksum is compared which was received and sent ==> Its the value is same then the data is not corrupted else it's corrupted
      • Timers:
        • If the data packets get lost during transmission, then timers help to know about it
        • "you can google about timers to know more"
      • Always Remember:
        • Transport layer protocols: TCP & UDP
        • Application layer protocols: HTTP
        • Network layer protocol: IP
      • UDP:(User Datagram Protocol)
        • It's a transport layer protocol
        • Data may/may not be delivered
        • Data may change on the way
        • Data may not be in order
        • UDP is a connectionless protocol: Which means no connection will be established between the two computers but it will still send the data
        • UDP uses checksums: So you will know whether the data is corrupted or not
        • but UDP will not do anything about it
        • We use UDP because =>
          • it's a lot faster
          • Used in video calling apps
          • DNS used UDP
      • TCP:(Transmission Control Protocol)
        • It's a transport layer protocol
        • HTTP uses TCP
        • Data is not lost
        • Mechanism:
          • The application layer sends lots of raw data --- TCP segments this data (divide in chunks, add headers etc)
        • TCP maintains the order of data using sequence no.
        • Features of TCP:
          • Connection-oriented
          • Full duplex
          • Error control
          • Congestion control
      • 3-Way Handshake:(for TCP connection)
        • This could also be seen as a way of how TCP connection is established
        • 
        • The 3-Way Handshake process is the defined set of steps that takes place in the TCP for creating a secure and reliable communication link and also closing it.
        • Machenism:
        • 
        • Step 1 (SYN): In the first step, the client wants to establish a connection with a server, so it sends a segment with SYN(Synchronize Sequence Number) which informs the server that the client is likely to start communication and with what sequence number it starts segments with
        • Step 2 (SYN + ACK): Server responds to the client request with SYN-ACK signal bits set. Acknowledgement(ACK) signifies the response of the segment it received and SYN signifies with what sequence number it is likely to start the segments with
        • Step 3 (ACK): In the final part client acknowledges the response of the server and they both establish a reliable connection with which they will start the actual data transfer

    • NETWORK LAYER:

      • Here we work with routers
      • Every single router has its own network address
        • Network address allows us to send the data packets to a router(lets say N1)
        • This N1 is going to check its routing table
          • Routing table consists of every destination address
            • Control plane Control plane is used to build these routing tables
            • Two types of routing:
              • Static routing: where we add address manually
              • Dynamic routing: This sort of evolves on its own ---> Algos like Dijkstra's etc are used
          • Forwarding table is a subset of the routing table
      • IP:(Internet protocol)
        • Its a network layer protocol
        • Here we work with how the routing, control plane stuffs etc happen.
          • IP address:
            • IPV4 => These are 32 bit nos. with 4 words
              • e.g.->
        • 
          • Good to know stuff:
            • Earlier assigning of IP address used to happen on the basis of class(class A,B,C,D,E) but now it happens on the basis of regions
          • IPV6 => These are 128 bits nos.
            • It is 4 times larger than IPV4
            • Cons:
              • Not backward compatible
              • ISPs would have to shift= Lot of hard work is needed
      • Subnet Masking:
        • Subnet masking means: It is going to mask the network part of the IP address and it will leave us to use the host part(device part)
      • Middleboxes
        • These are extra devices that also interact with IP packets
          • Firewall:
            • It filters out the IP packets based on various rules:
              • Address
              • Port nos.
              • Flags
              • Protocols
              • Two types of firewalls:
                • 1. Stateless
                • 1. Statefull--> More efficient

    • DATA-LINK LAYER:

      • The datalink layer is responsible to send the packets(received from network layer) over a physical layer i.e. transport data between connected devices