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Networking

Modem connects to internet. Router connects devices to modem.

graph LR;

router[Router
DHCP Server]

d1[Laptop
Pvt IP: 192.168.1.10] <--> router
d2[Mobile
Pvt IP:  192.168.1.22] <--> router
d3[TV
Pvt IP:  192.168.1.14] <--> router
d4[Tablet
Pvt IP:  192.168.1.12] <--> router

router <--> Modem[Modem
Public IP
82.129.70.44]

Modem <--ISP--> Internet

subgraph LAN[Local Area Network]
  d1
  d2
  d3
  d4
  router
end

Router lets you connect multiple devices together to share internet connection. It gives each device a private IP address like 192.168.1.xxx. The router (or modem) itself has a public IP address like 82.129.70.xxx which is given by ISP. So your device requests are uniquely addressed on internet by the comnination of both public and private addresses.

No two routers on the internet can have the same Public IP address.

Eg, Your flat has a wifi network Wifi-Flat-101 has public IP 82.123.123.101 and has two devices connected, a laptop with private IP address 192.168.1.10 and a mobile with private ip address 192.169.1.14. You neighbour's flat will a separate network with say wifi name Wifi-Flat-105 and may have public IP address 82.123.123.205 and has a TV connected to wifi having private IP address 192.168.1.10. Notice, how in separate networks private IP can be same and how two public IP address on separate router can't be same.

Now these private IP address are not magically made available to any new device connecting to router. It is done by DHCP Server on the routers. It has an IP address and port and can be found using ipconfig /all on windows. DHCP Server's IP is like 192.168.1.1 and port 68.

Similarly router has DNS Server which lets map custom domain name to address. Eg, type http://hyperhub/ and you are taken to http://192.168.1.1:80/ which is GUI to manage router. DNS IP is like 192.168.1.1 and port 53.

DHCP Server - "A DHCP Server is a network server that automatically provides and assigns IP addresses, default gateways and other network parameters to client devices". If you do not enable DHCP on router, then only one machine can connect to internet.

DNS Server or domain name server lets map domain name to IP address and port.

Localhost or 127.0.0.1 is the IP address of local computer that is the one you are working on and is used by machine to connect and communicate with itself.

A machine can have more than one IP address and can be done using multiple network card.

Starting a server means it can listen to requests. It will listen to a particular Address and Port. So if a server is started at 127.0.0.1:5000 it listens to request from local machine only.

if a server is started at 0.0.0.0:5000 it listens to request from all IP addresses a machine has. Usually machine has a private IP address if connected to router and an address of itself. So this server will listen to requests on 127.0.0.1:5000 and say 192.168.1.15.

2025-01-12 Aug 2023