Previously, there existed Circuit Switching → where the path (Bandwidth)/resources essential for the transmission are preserved prior to the actual transmission, Costly!!, Limited Infra!!

So, Packet Swicthing → Data is broken down to packets and sent from Source to Destination Then how to make sure correct and successful delivery of packets without/less packet loss ?

TCP/IP?

Transmission Control Protocol → ensures that there happens errorless delivery and assemble of packets.

Internet Protocol → ensures that the packet is delivered to the right destination/host.

So it’s as this, Application Layer : The messag e/data is being prepared here. The URL gets converted to Domain Name. Ex : HTTP, FTP Presentation Layer : This layer is responsible for formatting (.php, .js, .png) of the payload/data and does the encryption as well. Ex : .php, .js, .html, .png, .mov Session Layer : This Layer is responsible for Creating, Managing, Terminating of Sessions Ex : NETBios → To share file over NW By creating sessions :)

(SSL/TLS operates here for encrypted HTTPS sessions, not performing the 3-way handshake but encrypting data once TCP establishes the connection.)

Transport Layer : Is responsible for end-end delivery, doesn’t know/bother about the NW path in btw the source and destination. Adds Port Numbers of both the destination and Source to ensure the delivery of the packets to the exact process at the Destination machine. A logical Connection btw 2 processes on two hosts, hence needs adding of P# to identify exact process Has TCP, UDP to achieve these deliveries. Does TCP 3-Way Handshake, Flow-Control, Windowing, Congestion control via Buffering and is a heart of the Network. Ex : TCP, UDP Unit of Transmission : Segments

Network Layer : Responsible for the Hop-Hop or Routing btw Source → Destination. Here the IP Addresses of the src & destn are added on to the packets. A logical communication between 2 Hosts Does DNS Lookups. Routers exist here and does Routing and Packet switching Y& Circuit Switching if necessary Ex : Routers, IPv4, IPv6, ICMP. Unit of Transmission : Packets

Data Link Layer : Responsible for identifying the processes uniquely at both ends and deliver to the right destination. So adds the MAC Address which can be found on NIC(NW Interface Card). Packages data into frame and transmits. Has Switches and Does Actual Transmission on links! Ex : MAC(NIC), Switches. Unit of Transmission : Fragments

Physical Layer: Responsible for the actual physical connection (cables, signals, etc.).

Ex: Cables, Wi-Fi, Ethernet

DHCP → D-O-R-A

DORA : Discover — Offer — Request — Acknowledge The term DORA in the context of DNS stands for the DORA process, which refers to the four key steps in the DHCP (Dynamic Host Configuration Protocol) operation for obtaining network configuration. These four steps are:

1. Discover:
   • The process begins when a client (e.g., a computer or a mobile device) connects to a network and tries to obtain an IP address. Since it doesn't have an IP address yet, it broadcasts a DHCP Discover message to all devices on the network. This message essentially says, "I need an IP address."
2. Offer:
   • Upon receiving the DHCP Discover message, DHCP servers on the network respond by sending a DHCP Offer message back to the client. This message contains an available IP address, subnet mask, default gateway, and other network settings. If there are multiple DHCP servers, the client may receive multiple offers.
3. Request:
   • After receiving one or more DHCP Offers, the client chooses one (usually the first one it receives) and sends a DHCP Request message back to the DHCP server that made the offer. This message indicates that the client accepts the IP address and network configuration provided in the offer.
4. Acknowledge:
   • Finally, the chosen DHCP server sends a DHCP Acknowledge (ACK) message to the client. This message confirms that the client can use the provided IP address and network configuration. Once the client receives this acknowledgment, it officially adopts the network settings and becomes a part of the network.

DHCP Server runs at Port Number : 67

DHCP Client runs at Port Number : 68