IPv4 Addressing & Subnetting

🧠 COMPREHENSIVE NOTES ON IPv4 ADDRESSING & SUBNETTING

🧩 I. FUNDAMENTALS OF IP ADDRESSING

🔹 A. Why IP Addresses Are Needed

• The Internet Protocol (IP) assigns unique identifiers to devices so they can exchange data.
• In a simple two-device (point-to-point) link, identifiers aren’t needed since data flows directly.
• In any network with three or more devices, unique addresses are essential for correct delivery.

🔹 B. Address Function Across OSI Layers

Layer	Type of Address	Description
Layer 2 (Data Link)	MAC Address	Unique per device; doesn’t indicate location (like a name).
Layer 3 (Network)	IP Address (IPv4 / IPv6)	Indicates both who and where a device is located.

When data moves across a network, IP encapsulates it in a packet, tagging both:

• Source IP: Origin of the packet
• Destination IP: Target device’s address

🌐 II. IPv4 ADDRESS STRUCTURE

• Total Length: 32 bits
• Human Representation: Four octets in dotted-decimal form (e.g., 192.168.1.10)
• Binary Division: 4 octets × 8 bits = 32 bits

Each IPv4 address has:

1. Network Portion: Identifies the network segment (shared by all devices in that subnet).
2. Host Portion: Uniquely identifies the device within that network.

🧮 III. ROLE OF THE SUBNET MASK

🔹 A. Function of a Subnet Mask

• Definition: A 32-bit number that determines which part of the IP address represents the network vs. the host.
• Display: Dotted-decimal form (e.g., 255.255.255.0).
• Rules of Comparison:
  • 255 → Corresponding bits are network bits.
  • 0 → Corresponding bits are host bits.

🔹 Routing Logic

• If the destination is within the same subnet, packets are sent directly.
• If outside, they’re forwarded to the default gateway (router).

🔹 B. IPv6 Comparison

Feature	IPv4	IPv6
Address Length	32 bits	128 bits
Format	Dotted-decimal	Hexadecimal with colons
Subnet Notation	Mask or / slash (e.g., /24)	Always uses / notation (e.g., /64)
Parts	Network + Host	Prefix + Interface ID

🌍 IV. TYPES OF IPv4 ADDRESSES

🔹 A. Public vs. Private

Type	Purpose	Address Range
Public	Globally unique, routable on the Internet. Any address not in private range (1.0.0.0–9.x.x.x, 11.x.x.x–126.x.x.x, 128.x.x.x–223.x.x.x).	—
Private	Used only within local networks. Not Internet-routable.	10.0.0.0 – 10.255.255.255 172.16.0.0 – 172.31.255.255 192.168.0.0 – 192.168.255.255

🔹 B. Reserved IPv4 Addresses

Type	Rule	Example
Network Address	All host bits = 0	20.1.17.0/24
Broadcast Address	All host bits = 1	20.1.17.255
Limited Broadcast	Reaches all local devices; not routed beyond local network.	255.255.255.255

⚠️ Note: IPv6 does not use broadcast addresses.

🖥️ V. ADDRESS ASSIGNMENT METHODS

Method	Description
DHCP (Dynamic)	Device requests IP, subnet mask, gateway, and DNS from DHCP server.
Static	Manually assigned and unchanging; common for servers and routers.
APIPA (Automatic Private IP Addressing)	Self-assigned IP (169.254.x.x) when DHCP fails — indicates a connectivity issue.

🧱 VI. SUBNETTING

🔹 A. Why Subnetting Is Used

• To segment large networks for better security, management, and traffic control.
• Example: Separating HR, Finance, and Engineering into their own subnets.

🔹 B. How Subnetting Works

• Subnetting involves extending the subnet mask by converting host bits into network bits.
• This allows multiple smaller subnets under one larger parent network.

🔹 C. Subnet Calculation (The “Finger Method”)

1. Determine the required number of subnets.
2. Use powers of two to calculate how many bits to borrow:
   • 1 bit → 2 subnets
   • 2 bits → 4 subnets
   • 3 bits → 8 subnets
   • 4 bits → 16 subnets
   • 5 bits → 32 subnets
3. Add the borrowed bits to the existing mask to find the new subnet mask.

🔹 D. Special Case: /31 Subnet Mask

• Used for point-to-point links (two devices only).
• Leaves just 1 host bit, enabling exactly 2 usable addresses.
• Routers treat both addresses as valid (breaking the typical “no all-zeros/all-ones” rule).

⚙️ VII. RELATED NETWORKING CONCEPTS

Concept	Description
DNS (Domain Name System)	Translates human-readable names into IP addresses. Example: 8.8.8.8 (Google DNS).
Wireshark	Packet capture and analysis tool showing real-time source and destination IPs.
Wildcard Mask	Used mainly in ACLs (Access Control Lists). Reverse of a subnet mask: 0 = must match, 1 = ignore bit. Example: Subnet Mask 255.255.255.0 → Wildcard Mask 0.0.0.255.

💡 QUICK RECAP: IPv4 Essentials for Interview & Revision

✅ IPv4 = 32-bit addressing system using dotted-decimal format.
✅ Subnet Mask determines Network vs. Host bits.
✅ Private IPs are for internal use only.
✅ Broadcast & Network addresses are reserved.
✅ Subnetting enhances efficiency, control, and security.
✅ /31 mask is optimized for router-to-router links.
✅ DHCP automates IP assignment; APIPA indicates DHCP failure.
✅ DNS resolves names; Wireshark inspects packets.
✅ Wildcard Masks are used for security filtering.

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📘 Created for:

• Learners preparing for networking certifications (CCNA, CompTIA, etc.)
• Interviewers seeking precise technical evaluation points
• Engineers wanting quick refreshers on IPv4 and subnetting fundamentals

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