--- title: "Subnetting for Banijya bank departments" subject: "Computer Networks and Security" chapter: 4 key: "semester_7/computer_networks_and_security:subnetting-banijya-bank" source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/subnetting-banijya-bank site: BCT Engineering --- # Subnetting for Banijya bank departments Computer Networks and Security · Chapter 4: Network Layer - Asked in: 2069 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/subnetting-banijya-bank - Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-4 ## Question What is the contribution of sub-netting in IP address management? Show the importance in this case. Banijya bank needs to allocate 15 IPs in the HR department, 30 in the finance department, 24 in the customer care unit and 25 in ATM machines. If you have one network of class C range public IP address, describe how you will manage it. ## Short answer Subnetting reduces wastage by tailoring subnet sizes. Using VLSM, all /27 blocks: Finance 200.1.1.0/27, ATM 200.1.1.32/27, Customer care 200.1.1.64/27, HR 200.1.1.96/27. Total wastage 26 host addresses. ## Answer **Contribution of subnetting.** Subnetting divides one network into smaller subnets by borrowing host bits. It reduces address wastage (each department gets a block of the right size), limits broadcast traffic to each subnet, improves security (departments can be separated by router/ACL), simplifies fault finding and management, and allows one public network to serve many departments. For a bank, ATM, finance and HR traffic can be kept apart. Assumption: the Class C public block is 200.1.1.0/24. Note that 15 hosts need 17 addresses, so a /28 (14 hosts) is too small and a /27 is required. **Method (VLSM):** each department needs *hosts + 2* addresses (network address and broadcast address cannot be given to hosts). Choose the smallest block 2ⁿ ≥ hosts + 2, where n is the number of host bits and prefix = 32 − n. Allocate the **largest block first**, one after another without overlap, so every block starts on a boundary that is a multiple of its size and the waste is minimum. #### Step 1: Block size for each department | Dept | Hosts | Hosts + 2 | Block 2ⁿ | n | Prefix | Subnet mask | |---|---|---|---|---|---|---| | HR | 15 | 17 | 32 | 5 | /27 | 255.255.255.224 | | Finance | 30 | 32 | 32 | 5 | /27 | 255.255.255.224 | | Customer care | 24 | 26 | 32 | 5 | /27 | 255.255.255.224 | | ATM | 25 | 27 | 32 | 5 | /27 | 255.255.255.224 | #### Step 2: Allocation (largest first) from 200.1.1.0/24 | Dept | Network address | Usable host range | Broadcast address | Mask | Unused hosts | |---|---|---|---|---|---| | Finance | 200.1.1.0/27 | 200.1.1.1 – 200.1.1.30 | 200.1.1.31 | 255.255.255.224 | 0 | | ATM | 200.1.1.32/27 | 200.1.1.33 – 200.1.1.62 | 200.1.1.63 | 255.255.255.224 | 5 | | Customer care | 200.1.1.64/27 | 200.1.1.65 – 200.1.1.94 | 200.1.1.95 | 255.255.255.224 | 6 | | HR | 200.1.1.96/27 | 200.1.1.97 – 200.1.1.126 | 200.1.1.127 | 255.255.255.224 | 15 | #### Step 3: Wastage | Dept | Usable addresses | Needed | Wasted | |---|---|---|---| | Finance | 30 | 30 | 0 | | ATM | 30 | 25 | 5 | | Customer care | 30 | 24 | 6 | | HR | 30 | 15 | 15 | | **Total** | 120 | 94 | 26 | Addresses allocated: 128 of 256. The remaining 128 addresses (200.1.1.128/25) stay free for future departments and are not counted as wastage. **Answer: HR: 200.1.1.96/27; Finance: 200.1.1.0/27; Customer care: 200.1.1.64/27; ATM: 200.1.1.32/27.** Total wastage = 26 host addresses. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a benefit of subnetting? A. It increases total address space B. It merges all departments into one broadcast domain C. It limits broadcast traffic to each subnet and reduces wastage D. It removes the need for routers Answer: C. Subnetting limits broadcasts, cuts wastage and lets departments be separated by router/ACL. (Key point: Benefits of subnetting) 2. Why can HR (15 hosts) not use a /28? A. A /28 is a Class B mask B. HR must share with finance C. 15 hosts need a /26 D. A /28 has only 14 usable hosts Answer: D. 15 + 2 = 17 > 16, so a /27 is required. (Key point: HR block size) 3. Which department is allocated first at 200.1.1.0/27? A. HR (15 hosts) B. Finance (30 hosts) C. Customer care (24 hosts) D. ATM (25 hosts) Answer: B. Finance has the most hosts, so it takes the first block. (Key point: Largest first) 4. What is the total wastage? A. 128 B. 26 C. 94 D. 120 Answer: B. 0 + 5 + 6 + 15 = 26. 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