VPN & Types

Secured tunnel (protected network) between different networks using the internet (public network)

Access VPN: Provide connectivity to remote mobile users and telecommuters Site to Site VPN: Large companies having branches in different locations to connect the network of one office to another in different locations. It can be Intranet or Extranet

SMTP

Send-to-End and Store-and-Forward methods. Default: port 25.
Protocol for Mail Transfer across servers.

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OSI Model & TCP/IP

Note

Transport Layer is the heart of OSI

OSI - 7

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ARP

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What happens when you enter google.com in the web browser?

Check the browser cache first if the content is fresh and present in cache display the same.
If not, the browser checks if the IP of the URL in cache (browser and OS) if not then request the OS to do a DNS lookup using UDP to get the corresponding IP address of the URL from the DNS server to establish a new TCP connection.
A new TCP connection is set between the browser and the server using three-way handshaking.
An HTTP request is sent to the server using the TCP connection.
The web servers running on the Servers handle the incoming HTTP request and send the HTTP response.
The browser process the HTTP response sent by the server and may close the TCP connection or reuse the same for future requests.
If the response data is cacheable then browsers cache the same.
Browser decodes the response and renders the content.

Router vs Gateway

Router: Directs the traffic in the network from source to destination in the form of packets. It operates at the [network layer]
Gateway: The gateways are also used to route and regulate the network traffic but, they can also send data between two dissimilar networks

While a router can only send data to similar networks.

Default PORTS

HTTP: 80
HTTPS: 443
SSL: 443 (commonly used with HTTPS)
SMTP: 25
POP3: 110
IMAP: 143
FTP: 21 (control), 20 (data)
SSH: 22
Telnet: 23
DNS: 53
DHCP:
- 67 (server)
- 68 (client)
RDP: 3389
MySQL: 3306
PostgreSQL: 5432
ICMP: 7

ICMP

Internet Control Message Protocol.

Network layer protocol used for error handling.

Used by network devices like routers for diagnosing the network connection issues and crucial for error reporting