IP Addressing, CIDR, VLSM: Subnetting & Efficient Network Design

TAKEAWAYS:

• IP addresses are hierarchical identifiers (network + host) with classes (A/B/C/D/E) and modern CIDR notation (e.g., 192.168.1.0/24).
• CIDR replaces classful addressing by using variable-length subnet masks (e.g., /24 for 256 hosts) and supernetting (e.g., 10.0.0.0/8 for large networks).
• VLSM enables flexible subnet sizing (e.g., /30 for point-to-point links, /26 for LANs) to conserve IP space and reduce waste.
• Subnetting divides networks into smaller subnets using borrowed bits (e.g., /24 → /26 + /28 for different needs).
• Real-world applications include eSewa’s payment routing (subnetting for security zones), Ncell’s core networks (VLSM for efficient IP allocation), and home routers (DHCP + CIDR for local addressing).
• Exam focus: calculations (subnet masks, usable hosts), CIDR vs. VLSM comparisons, and troubleshooting (e.g., misconfigured subnets causing connectivity issues).

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1. IP Addressing Basics: Classes and Binary Representation

IPv4 addresses are 32-bit binary numbers divided into network and host portions. Historically, addresses were classified into A, B, C, D, E based on the first few bits:

Why classes matter (but are outdated):

• Class A: Large networks (e.g., 10.0.0.0/8 for private networks).
• Class B: Medium networks (e.g., 172.16.0.0/16).
• Class C: Small networks (e.g., 192.168.1.0/24 for home routers).
• Problem: Wasted IPs (e.g., Class A gives 16M hosts when only 100 are needed).

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2. CIDR: Classless Inter-Domain Routing

CIDR replaces classful addressing by using variable-length subnet masks (e.g., /24, /16). Key concepts:

CIDR Notation

• 192.168.1.0/24 means:
  • Network: 192.168.1.0 (first 24 bits fixed).
  • Hosts: Last 8 bits (192.168.1.1 to 192.168.1.254).
  • Usable hosts: 2^(32-n) - 2 (subtract network and broadcast addresses).

Supernetting (Aggregation)

Combines multiple networks into one larger prefix to reduce routing table entries. Example:

• 192.168.1.0/25 and 192.168.1.128/25 → 192.168.1.0/24 (single route).

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3. VLSM: Variable Length Subnet Masking

VLSM allocates subnets of different sizes from a single IP block to conserve addresses. Used when:

• Some subnets need few hosts (e.g., point-to-point links: /30).
• Others need many hosts (e.g., LANs: /24).

How VLSM Works

1. Start with a large block (e.g., 10.0.0.0/16).
2. Borrow bits from the host portion to create smaller subnets.
3. Assign subnets based on need (e.g., /30 for routers, /26 for departments).

Worked Example: Ncell’s Core Network Ncell uses VLSM to allocate IPs to:

• Backbone links (/30 subnets, 2 hosts each).
• Regional offices (/26 subnets, 62 hosts each).
• Mobile towers (/31 subnets, 0 hosts—used for router adjacency).

Calculation Steps:

1. Start with 10.0.0.0/24 (256 addresses).
2. Allocate /30 for backbone (4 subnets: 10.0.0.0/30, 10.0.0.4/30, etc.).
3. Remaining space: 10.0.0.8/29 → Split into /26 for offices.

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4. Subnetting: Step-by-Step

Step 1: Determine Required Hosts per Subnet

Step 2: Calculate Subnet Mask

• Formula: Subnet Mask = 32 - (required hosts + 2) (Add 2 for network/broadcast addresses.)
• Example: For 30 hosts → /27 (32 - 5 = 27).

Step 3: Draw the Subnet

Example: Subnet 192.168.1.0/26 (62 hosts).

• Network: 192.168.1.0
• Broadcast: 192.168.1.63
• Usable hosts: 192.168.1.1 to 192.168.1.62

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5. CIDR vs. VLSM: Comparison

Real-World Tie-In:

• eSewa uses CIDR for payment gateway routing (e.g., 10.0.0.0/8 for internal servers).
• Ncell uses VLSM to segment mobile towers (/31 for adjacency, /28 for base stations).

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6. Common Mistakes and Fixes

Exam Pitfall:

• Forgetting to subtract 2 for network/broadcast addresses.
  • ❌ 2^8 = 256 hosts (wrong).
  • ✅ 2^8 - 2 = 254 hosts (correct).

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## In the Real World

1. eSewa’s Payment Routing

   • Uses CIDR blocks (10.0.0.0/8) to segment:
     • User databases (/24 subnets).
     • Payment gateways (/26 subnets for security).
   • Why? Isolates critical systems and reduces attack surface.

2. Ncell’s Mobile Backbone

   • Applies VLSM to allocate:
     • /30 subnets for fiber links between towers.
     • /28 subnets for base station clusters.
   • Result: Saves ~40% IPs vs. fixed-length subnetting.

3. Home Router (e.g., TP-Link)

   • Assigns 192.168.1.0/24 via DHCP.
   • Uses VLSM internally to manage:
     • LAN (/24).
     • Guest network (/26).
     • IoT devices (/30 for point-to-point).

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## Exam Tip

1. Memorize Key CIDR Blocks:

   • /30: 2 hosts (point-to-point).
   • /28: 14 hosts (small office).
   • /24: 254 hosts (LAN).
   • /16: 65K hosts (large enterprise).

2. Practice Calculations:

   • Given: 172.16.0.0/20 → Find subnets. Steps:
     1. Borrow 4 bits → 172.16.0.0/24, 172.16.1.0/24, ..., 172.16.15.0/24.
     2. Usable hosts per subnet: 254.
   • Given: Need 5 subnets with 30 hosts each → /27 (30 hosts + 2 = 32 → 2^5 = 32 subnets).

3. Diagrams Are Worth Marks:

   • Draw subnet breakdowns (like the /26 example above).
   • Label network, broadcast, and usable hosts.

4. Common Exam Questions:

   • "Convert 255.255.255.192 to CIDR" → /26.
   • "Why use VLSM?" → Conserves IPs, reduces waste.
   • "What’s the issue with 192.168.1.0/25?" → Only 126 usable hosts (not enough for a LAN).

5. Real-World Scenario (NEPSE Stock Exchange):

   • NEPSE’s trading servers use 10.10.0.0/16 with VLSM:
     • /28 for trading terminals (14 hosts).
     • /30 for inter-server links.
   • Question: "How many subnets can be created from 10.10.0.0/20 for 20 hosts each?" Answer: Borrow 6 bits → 2^6 = 64 subnets.

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