1.Modem

Modem is used to MOdulate and DEModulate singal. It is a device which is used to connect local network to the Internet. it directly connects with the Internet Service Provider(ISP).
It is a gateway between network and internet.

Why Modem is required?

ISPs transmit data over telephone lines, cable lines or fiber. These lines use different signal format.
The modem converts signals so both sides can understand each other as Computers and routers work with digital signal.

How a Modem connects network to the internet?

ISP sends internet signals to the location. The modem receives the signal and convert it into digital data. The Data is passed to a router and router distributes internet access to devices in your network.

Modem: Connects network to the ISP.
Router: Shares the internet within network.
Modern devices are combination of both function.

2.Router

Router is a networking devices that connects multiple network together. In home and offices, it connected local network to the internet working at the NETWORK LAYER(layer-3) of the OSI model.
Router acts as a traffic manager, deciding where data should go.

How does a Router directs Traffic?

Every data packet has Source IP Address and Destination IP address. The Router reads the destination IP and decides the best path and forwards the packet to the next network or device. This process is known as "routing”.

What is Routing Table?

A routing table is a database inside a router that contains Destination address.

3. Switch vs Hub: how local network actually work?

A Local Network(LAN) is used to connect multiple devices allowing them to share the data avoiding unnecessary traffic.
Early LANs were simple known as hub.
Modern LANs are intelligent knowns as switch.

What is HUB?

HUB acts as a central connection point for devices in a local area network.
How does it send data?
It receives data from one port and sends it all ports.
It does not understand addresses and does not filter traffic.
It works at physical layer(layer 1) on OSI model.

How does the traffic flow?

A device sends the data which is received by HUB. HUB broadcasts it to all devices and every devices needs to check if data is for it that results in Network Congestion, collision of data
e.g., A loudspeaker announce a message to everyone.

What is Switch?

A Switch is a hardware device that connects multiple devices within Local Area Network and intelligently forwards data packets only to the intended recipient.
How does it send data?
It has a ability to read MAC addresses.
A Switch can build a MAC address table and forward the data only where it is needed.
Switch does not talk to internet and does not choose routes across network.
Switch works at Data Link Layer(Layer 2).

How does the traffic flow?

Device sends data and Switch reads destination MAC address. It checks MAC table and data is forwarded to the correct port that results into Faster communication and no unnecessary traffic. It actually helps us to build a secure and scalable network.
e.g., An office receptionist delivering mail to right desk.

4.What is a Firewall and why security lives here?

A firewall is the first security decision point between trusted and untrusted networks.
The Primary task of the firewall is to allow trusted traffic and block untrusted traffic.
Internet → Firewall → Router/Switch → Internal Network/Servers
A firewall inspects network traffic and applies security.
e.g., Some are allowed and some are denied at the security gate of the building.

How does a Firewall actually work?

To determine whether to allow or deny a packet, the firewall evaluates specific attributes : the source and destination IP addresses, the designated port and protocol, and the packet’s connection state.

Where does the security live?

Security lives at the firewall because this is where control meets visibility. All traffic must pass through firewall. It prevents Unauthorised access.

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