IP Addressing

1. IP Address Basics

Definition: A logical identifier assigned to a device for communication over a network.

Property IPv4 IPv6
Bits 32 128
Format Decimal (4 octets) Hexadecimal
Example 192.168.1.10 2001:0db8:85a3::8a2e:0370:7334
Subnetting ✅ Yes ✅ Yes (advanced)
Used today ✅ Widely ✅ Growing

IPv4 Structure: 

8 bits | 8 bits | 8 bits | 8 bits  →  4 octets
Min per octet = 0,  Max per octet = 255  (2⁸ = 256 values)

2. Binary Conversion (Why It Matters)

Computers work in binary; IP math always reduces to binary.

Powers of 2 (memorize this row): 

128  64  32  16  8  4  2  1

Example — Convert 192: 

192 ≥ 128 → 1,  remainder 64
64  ≥ 64  → 1,  remainder 0
Rest       → 0

Result: 11000000

3. IP Classes (Legacy / Classful — Conceptual Only ⚠️)

Classes are obsolete in production. Learn for interviews, think in CIDR for real work.

Class First Octet Range Leading Bits Default Prefix Usage
A 0–127 0 /8 Large networks
B 128–191 10 /16 Medium networks
C 192–223 110 /24 Small networks
D 224–239 1110 — Multicast
E 240–255 1111 — Reserved/Experimental

Special Ranges (Never assign to hosts): - 0.x.x.x → Reserved (network zero) - 127.x.x.x → Loopback (127.0.0.1 = localhost)

Rule: Class is determined by the first octet only.

4. Network vs Host (Critical Concept)

Every IP = Network part + Host part

Class Structure Network Bits Host Bits
A N . H . H . H 8 24
B N . N . H . H 16 16
C N . N . N . H 24 8
  • Network bits → identify which group/network
  • Host bits → identify which device inside that group

5. Network ID & Broadcast Address

Type Rule Purpose
Network ID (First IP) Set all host bits = 0 Represents the network, cannot assign to host
Broadcast (Last IP) Set all host bits = 255 Sends packet to ALL devices in network

Examples:

Given IP Class Network ID Broadcast
10.20.30.40 A 10.0.0.0 10.255.255.255
172.16.5.4 B 172.16.0.0 172.16.255.255
192.168.1.50 C 192.168.1.0 192.168.1.255

6. Usable IPs Formula

Total IPs  = 2^(host bits)
Usable IPs = Total - 2        ← subtract Network ID + Broadcast
Class Host Bits Total Usable
A 24 16,777,216 16,777,214
B 16 65,536 65,534
C 8 256 254

7. CIDR — Classless Inter-Domain Routing (Modern System ✅)

Format: 192.168.1.0/24 /24 = 24 bits for network, remaining (32-24) = 8 bits for host.

Total IPs = 2^(32 − CIDR prefix)
CIDR Host Bits Total IPs Usable IPs
/8 24 16,777,216 16,777,214
/16 16 65,536 65,534
/24 8 256 254
/25 7 128 126
/26 6 64 62
/28 4 16 14
/30 2 4 2
/32 0 1 1 (single host)

Rule: Bigger prefix / = smaller network. Smaller prefix / = bigger network.

Subnet Mask

A subnet mask is a 32-bit number that separates network bits (all 1s) from host bits (all 0s).

CIDR Subnet Mask Binary
/8 255.0.0.0 11111111.00000000.00000000.00000000
/16 255.255.0.0 11111111.11111111.00000000.00000000
/24 255.255.255.0 11111111.11111111.11111111.00000000
/26 255.255.255.192 11111111.11111111.11111111.11000000
/28 255.255.255.240 11111111.11111111.11111111.11110000

8. Subnetting

Definition: Dividing one large network into multiple smaller networks by borrowing host bits and giving them to the network.

Class A default:  N . H . H . H   (/8)
After /16:        N . N . H . H   (/16)  ← borrowed 8 bits from host
Effect Result
Borrow host bits → network More networks, fewer hosts per network
More subnets Better traffic control, security isolation

Example: 10.0.3.4/16 - Class A default = /8 - Given /16 → borrowed 8 bits → smaller network carved out of Class A space

9. Communication Rule

Step-by-step to check if two IPs can communicate:

  1. Find class (first octet)
  2. Apply network structure → find Network ID
  3. Compare Network IDs
IP Network ID Same Network?
192.168.1.10 192.168.1.0 ✅
192.168.1.15 192.168.1.0 ✅
192.168.1.12 192.168.1.0 ✅
192.168.2.11 192.168.2.0 ❌ Different

Same Network ID → Switch. Different Network ID → Router.

10. Switch vs Router

Switch Router
Works on MAC address (Layer 2) IP address (Layer 3)
Connects Devices within same network (LAN) Different networks
When needed Same Network ID Different Network IDs

11. Communication Modes

Mode Traffic Pattern Example
Unicast One sender → One receiver Opening google.com
Multicast One sender → Specific group Zoom call, live stream
Broadcast One sender → All devices in network ARP request

⚠️ Broadcast is local network only — it does NOT travel across routers to the internet.

12. Private vs Public IP (RFC 1918)

Private IP Ranges (cannot route on internet):

Class CIDR Range
A 10.0.0.0/8 10.0.0.0 – 10.255.255.255
B 172.16.0.0/12 172.16.0.0 – 172.31.255.255
C 192.168.0.0/16 192.168.0.0 – 192.168.255.255

These ranges are defined by RFC 1918 (IANA standard).

Public IP: Any IP outside the above ranges — globally unique, routable on internet.

NAT (Network Address Translation)

Private IP → NAT (Router/Gateway) → Public IP → Internet

Without NAT, private IPs have no internet access.

IP Assignment Chain

IANA → RIR (e.g., APNIC for Asia-Pacific) → ISP → You

You don't buy IPs from IANA directly. Your ISP assigns your public IP.

13. Network Types (LAN / MAN / WAN)

Type Scope How Connected Internet Required?
LAN (Local Area Network) Single building / floor Switch ❌ No
MAN (Metropolitan Area Network) City-wide (multiple LANs) Fiber / leased lines Optional
WAN (Wide Area Network) Global (multiple networks) Router + Internet ✅ Yes

Key rule: Network type is defined by how networks are connected, not just physical distance. LAN can exist with or without internet. LAN = private IP and WAN = public IP is an oversimplification — not strictly correct.

14. Complete Mental Flow (Memorize This) ⭐

Given any IP, think in this order:

1. Identify class       → look at first octet
2. Network structure    → how many octets = network
3. Network ID           → zero out host bits
4. Broadcast            → max out host bits (255)
5. Total/Usable IPs     → 2^(host bits), minus 2
6. CIDR check           → is prefix = default? If not → subnetting
7. Compare two IPs      → same Network ID? → switch / router decision

15. Common Mistakes ✅

❌ Wrong ✅ Correct
IPv6 has no subnetting IPv6 supports subnetting (advanced)
Classes are used in modern networking Classes are obsolete — CIDR is used
All IPs in a range are usable Network ID + Broadcast = reserved (−2)
LAN always needs internet LAN works without internet
LAN = private IP, WAN = public IP Not strictly true; based on network boundaries
You buy public IP from IANA IANA → RIR → ISP → You
Broadcast reaches the whole internet Broadcast is limited to local network only

16. Interview Questions Checklist ✅

  • What is an IP address? IPv4 vs IPv6?
  • How is IPv4 structured? Why max 255 per octet?
  • What are IP classes and their ranges? (leading bits reason)
  • What are network bits vs host bits?
  • How do you find Network ID? Broadcast address?
  • Formula: Total IPs and Usable IPs
  • What is CIDR? How is it different from classful?
  • What is a subnet mask? Give examples for /24, /26, /28
  • What is subnetting? What does "borrowing bits" mean?
  • How do you check if two IPs can communicate?
  • Switch vs Router — when is each needed?
  • Unicast vs Multicast vs Broadcast
  • Private IP ranges (RFC 1918) — all three
  • What is NAT and why is it needed?
  • LAN vs MAN vs WAN
  • IANA → RIR → ISP → You (IP assignment chain)