Network Fundamentals:

A network consists of some network devices and their interconnection using some media like copper wire or optical fiber or wireless link. The network devices can be classified into the following categories:

End devices:
These are the devices that provides with human interface like PC, Servers, Workstation, Network Printer, Closed Circuit cameras etc.
Intermidiary Devices: These devices are used to connect and control the flow of data, find the optimum route for destination. Most common are Router, Switch, Hub, Wireless access points
Security Devices: These provides security and filtering of data like Firewall

LAN & WAN:
LAN is the network of some private PCs usually located at the same geographic are. WANs are connection of several LANs.

Internet & Intranet:
Internet is the interconnection of a lot of WANs in which data is publicly accessible. Intranet is the interconnection of some WANs of a company that is accessible only by the employees and permitted users.

Protocols:
When we interact with someone we use some rules so that the person at the other end understands. Similarly the network devices when communicating with each other need to abide by some rules. These rules are called Protocols. Protocols specify the structure of the data, how the devices communicate to share different routes, the error and service message provided to the user and the start and termination of a data session. Usually IEEE, IETF, RFCs and other international society defines the protocols so that all over the world the communication is possible.
The important thing is that protocol only provides the rules, not the process by which they are implemented.

Layered Models:
The total network process is divided into several layers. Each layer works independently, so if a network switches from IPv4 to IPv6 only the network layer changes, the rest remains the same.

There are two basic types of models:
TCP/IP model
OSI reference model

OSI model consists of seven layers:
• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical
TCP/IP model has 4 layers:
• Application (Application, Presentation & Session layer of OSI model)
• Transport
• Internet (Network Layer)
• Network Access (Data Link & Physical layer)

Network Layer:

The network layer gets transport layer segment, adds ip address of source and destination and creates a PDU called packet. The ip address is used to forward data at routers and distribute it among the users.
Network layer is connectionless, so it has no headache to see data is delivered or not. That’s the responsibility of the transport layer.

Two common protocols of network layer are
IPv4: Now used with 32bit ip address
IPv6: Limited usage, with 128 bit ip, it provides better security and supports more users than IPv4

Apart from ip address, some other information are added to network layer packet. They are TTL (time to live)= how many routers the packet can pass before they are dropped
TOS (Type of service)=the info that routers use to give priority to packets like VIPs
Transport layer Protocols: TCP: 06, UDP: 17
MF (More fragment): Several fragments need to be added to form the packet
DF (Don’t Fragment): Delicate packet, don’t make pieces.

A router is a layer 3 (network) device. When several networks are connected through routers, the routers exchanges information by routing protocol. So each router knows which network is connected to which router or the shortest path to a distant network. When a router gets a frame (layer 2 PDU) it decapsulates the frame and then checks the ip address in the packet and finds the network address by binary ANDing the ip address and the subnet mask. If the destined network is connected to the router it delivers the packet. Otherwise it encapsulates the packet again and resends the packet to another router which is the next hop for the desired network.