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OSI Model vs TCP/IP Stack

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven distinct layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer serves a specific purpose and interacts with the layers directly above and below it. The TCP/IP (Transmission Control Protocol/Internet Protocol) stack, on the other hand, is a more practical and widely used model that consists of four layers: Link, Internet, Transport, and Application. The TCP/IP stack is designed to be simpler and more efficient for real-world networking.

  • Link Layer: Corresponds to the OSI's Physical and Data Link layers. It handles the physical transmission of data over a network and manages the hardware addressing (MAC addresses)and DNS resolution for local network communication.
  • Internet Layer: Corresponds to the OSI's Network layer. It is responsible for logical addressing (IP addresses) and routing of data packets across networks.
  • Transport Layer: Corresponds to the OSI's Transport layer. It manages end-to-end communication, error checking, and flow control. This is where TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) operate.
  • Application Layer: Corresponds to the OSI's Session, Presentation, and Application layers. It provides protocols for specific applications, such as HTTP/HTTPS for web traffic, DNS for domain name resolution, and FTP for file transfers.

Hands-on Checklist

  • Identity: hostname -I (or ip addr show) — shows th e IP address(es) assigned to the host.
  • Reachability: ping <target> — tests the reachability of a target host and measures the round-trip time for messages sent from the originating host to a destination computer.
  • Path: traceroute <target> (or tracepath) — displays the route and measures transit delays of packets across an IP network.
  • Ports: ss -tulpn (or netstat -tulpn) — lists all listening ports and the associated services.
  • Name resolution: dig <domain> or nslookup <domain> — queries DNS servers to resolve domain names to IP addresses.
  • HTTP check: curl -I <http/https-url> — retrieves the HTTP headers from the specified URL, showing the HTTP status code and other metadata.
  • Connections snapshot: netstat -an | head — provides a snapshot of current network connections, showing the state of each connection (e.g., ESTABLISHED, LISTENING).

Mini Task: Port Probe & Interpret

i have tested on port no 80 its succesfull (test.png)

Reflection (add to your markdown)

  • Which command gives you the fastest signal when something is broken? curl -I as it will shows me the HTTP status code and headers.
  • What layer (OSI/TCP-IP) would you inspect next if DNS fails? If HTTP 500 shows up? applica1tion layer for both cases, as DNS is part of the application layer in the TCP/IP stack, and HTTP 500 is an error code that indicates a server-side issue, which also falls under the application layer.
  • Two follow-up checks you’d run in a real incident. dns failure, I would check the DNS server configuration and logs to identify any issues. Ports ss -tulpn to check if the DNS service is running and listening on the correct port (usually port 53).