1. Networking Fundamentals Master the OSI Model and TCP IP

OSI Model:-  Basic standards for network communication

Protocols 

Standards of communication : Packaging / Addressing / Payment 
/ Getting the package on the network
   
Layers: Application
        Presentation
        Session
        Transport
        Network
        Data Link
        Physical

Here remember layers numbered from bottom to top so Layer 3 means Network layer in OSI model.

 Function of each Layer

Application:- Deals with Network API's(Application programming interface's). It is an interface between your application and Operating Sytem. (Interfaces which communicates with your applications)

Presentation:- Deals with the formatting of the information that going out on the network.

Session:- synchronization / sending and receiving computer in synch with one another.

Transport:- Packet management, Data when it is sent, it is broken into packets and transport layer which manages those packets.
Keeping track of how many they are and did they get their.
              
Network:- Deals with addressing and routing, addressing with TCP/IP, And routing from one network to another.

Data:- Deals with data frames, frames means ethernet frames, Tocken ring frames etc. What kind of network we are using to package this datas.
               
Physical:- All about the Hardware, Network cards attached to the cables. an interface between application and OSI model.

 
  Sending Computer                  Receiving Computer
    Application     --------------     Application
       |                                  ^
       |                                  |
       >                                  |
    Presentation --------------        Presentation
       |                                  ^
       |                                  |
       >                                  |
    Session        --------------      Session
       |                                  ^
       |                                  |
       >                                  |
    Transport    --------------         Transport   
       |                                  ^
       |                                  |
       >                                  |
    Network        --------------       Network   
       |                                  ^
       |                                  |
       >                                  |
     Data        --------------         Data
       |                                  ^
       |                                  |
       >                                  |
    Physical    --------------         Physical   
       |                                  ^
       |                                  |
       |__________________________________|     
                   Network Cable
     

Arrows goes down through on sending side  And they go up through the OSI model on the Receiving side. that is important,

When sending some data, on sending side it starts from Application layer to Physical layer, and pick's up network cables and on Receiving end it goes up starting from Physical layer to Application Layer and each layer acknowledges what the corresponding layer on sending side added to the information.

When data goes down from application layer to physical layer while sending some data's will append to front or back ie, header or trailer of the data.  and it travels through Network cable and reaches to the Physical layer of the Receiving end and moves up to Application layer and each layer tripping off  the data of corresponding layer from the sending side has put on.

Device's in each Layer

Phyical Layer: USB, HUB, Bluetooth, NIC card
Data Link Layer: Switch(do filtering based on computers mac address)
Network: Router, IPV4, IPV6 and ICMP

 Rest three layers has no hardwares in it only protocols
 Transport: TCP, UDP
 Session: PPTP, TLS/SSL
 Presentation: MIME

Layer 3 lavel VPN Router:- 
         Network Layer, bcz layer counting from bottom to top.
       
 
 TCP/IP Model

Application: it will do the functionality of presentation and session layer of OSI model
Transport:
Internet:
Network Access:

 TCP/IP Protocols

Application:- HTTP, FTP, SNMP, DNS, SMTP
Transport:- TCP,UPD
Internet:- IP, ICMP,
Network Access:- Ethernet, Token Ring

TCP, Connection Oriented:- Connection Must be Established before data exchange, It will establish with something called Three-way Handshake
  
UDP, Connection less:- and un-reliable. Many used for Multi cast addressing ie, One computer is broadcasting and many listeners.

ICMP is used for Diagnostic and error reporting. icmp is used with UDP # ping

Protocol is a set of rules and procedure for communication.

 ThreeWay HandShake

 1.  The client sends a SYN packet to the server indicating
that it wants to set a TCP connection.It also sends ISN (Initial Sequenc Number). Here ISN is x.

2. If the server is 'alive' and listening on the requested
 port and can accept an incoming connection, it replies with its own SYN + ACK packet. It sends its own ISN (Initial Sequence Number)(for this connection, y ) and acknowledges the clients request by sending back  client's ISN + 1 sequence number (x + 1).

3. Finally, after receiving the server's SYN + ACK response, the client sends back an ACK packet with a sequence number of server's ISN + 1 (y + 1).

 In Some Easy Steps:-

  Host A sends a TCP SYNchronize packet to Host B
  Host B receives A's SYN
  Host B sends a SYNchronize-ACKnowledgement
  Host A receives B's SYN-ACK
  Host A sends ACKnowledge
  Host B receives ACK.
  TCP socket connection is ESTABLISHED.