RAKUB Network System Engineer (PO) Exam: 10.10.2021প্রশ্ন ও সমাধান

The TCP/IP model is the practical, 4-layer networking model that the actual Internet is built on — it predates and is simpler than the theoretical 7-layer OSI model, grouping related OSI layers together.

graph TD
  A[4. Application] --> T[3. Transport]
  T --> N[2. Internet]
  N --> L[1. Network Access / Link]

TCP/IP Layer	Role	Key Protocols	Roughly = OSI Layers
Application	User services, formatting, session handling	HTTP, FTP, DNS, SMTP	Application + Presentation + Session
Transport	End-to-end delivery, reliability	TCP, UDP	Transport
Internet	Logical addressing and routing	IP, ICMP, ARP	Network
Network Access (Link)	Physical transmission, framing	Ethernet, Wi-Fi	Data Link + Physical

Brief explanation of each layer:

• Application: where user applications and their protocols operate (e.g., a browser using HTTP).
• Transport: breaks data into segments and ensures reliable (TCP) or fast/best-effort (UDP) end-to-end delivery, using port numbers to distinguish applications.
• Internet: wraps segments into packets, attaches IP addresses, and routes packets across interconnected networks.
• Network Access: handles the physical transmission of raw bits over the actual medium (cable, radio) and framing at the hardware level.

Exam takeaway: TCP/IP has 4 layers (Application, Transport, Internet, Network Access) versus OSI's 7 — it is the model actually implemented by the real-world Internet protocol suite.

ARP (Address Resolution Protocol) maps a known IP address to the corresponding physical MAC (hardware) address on a local network, since Ethernet devices communicate using MAC addresses, not IP addresses directly.

sequenceDiagram
  participant H1 as Host 1 (192.168.1.1, MAC AAA)
  participant ALL as All hosts on LAN (broadcast)
  participant H2 as Host 2 (192.168.1.2, MAC BBB)
  H1->>ALL: ARP Request: "Who has 192.168.1.2?" (dest MAC FF:FF:FF:FF:FF:FF)
  H2-->>H1: ARP Reply: "192.168.1.2 is at MAC BBB"
  H1->>H1: Cache mapping (192.168.1.2 -> BBB) in ARP table

How it works: H1 broadcasts an ARP request to every host on the LAN; only the host whose IP matches (H2) replies directly with its MAC address; H1 stores this mapping in its local ARP cache to avoid repeating the broadcast for every packet.

Advantages:

• Automatically discovers MAC addresses — no manual configuration needed.
• Caching makes subsequent communication fast (no repeated broadcasts).

Disadvantages:

• Vulnerable to ARP spoofing/poisoning — a malicious host can send fake ARP replies claiming another host's IP, redirecting traffic (a classic man-in-the-middle attack vector).
• Broadcast-based, so it doesn't scale beyond a local subnet (routers don't forward ARP broadcasts).

Exam takeaway: ARP resolves IP→MAC via a local broadcast request/unicast reply, cached for efficiency — but its trust-everyone design makes it exploitable via ARP spoofing, which is why switches use features like Dynamic ARP Inspection (DAI) to defend against it.