Subnet Mask — Formal Length & Notation

Binary Mask Values and Formula

8-bit Binary Values

Each bit position in an 8-bit number represents a power of 2:

Bit Position	Value
7	128 (2^7)
6	64 (2^6)
5	32 (2^5)
4	16 (2^4)
3	8 (2^3)
2	4 (2^2)
1	2 (2^1)
0	1 (2^0)

Maximum 8-bit value:

11111111 = 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

Common Binary Masks

Binary	Decimal	Formula
11111111	255	256 – 1
11111110	254	256 – 2
11111100	252	256 – 4
11111000	248	256 – 8
11110000	240	256 – 16
11100000	224	256 – 32
11000000	192	256 – 64
10000000	128	256 – 128
00000000	0	256 – 256

Formula

For an 8-bit mask consisting of leading 1s followed by trailing 0s:

11111111
11111110
11111100
11111000
...

The decimal value is:

Value = 256 – 2^n

where:

• n = number of trailing zeros
• 256 = 2^8

Examples

11111100

Trailing zeros = 2

256 - 2²
= 256 - 4
= 252

11110000

Trailing zeros = 4

256 - 2⁴
= 256 - 16
= 240

11000000

Trailing zeros = 6

256 - 2⁶
= 256 - 64
= 192

10000000

Trailing zeros = 7

256 - 2⁷
= 256 - 128
= 128

00000000

Trailing zeros = 8

256 - 2⁸
= 256 - 256
= 0

Common Subnet Masks

CIDR	Dotted Decimal	Binary Representation	Usable Hosts	Typical Use
/8	255.0.0.0	11111111.00000000.00000000.00000000	16,777,214	Large private networks
/16	255.255.0.0	11111111.11111111.00000000.00000000	65,534	VPC CIDR block
/24	255.255.255.0	11111111.11111111.11111111.00000000	254	Standard subnet
/28	255.255.255.240	11111111.11111111.11111111.11110000	14	Small subnet

Formula: Usable Hosts = 2^(32 − prefix) − 5

Internet Protocol (IP) is the communication protocol used to identify and route devices across networks. The two major versions in use today are IPv4 and IPv6.

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IPv4

Definition

Internet Protocol Version 4 (IPv4) is the fourth version of the Internet Protocol and has been the primary protocol used on the Internet for decades.

Address Format

IPv4 addresses are 32-bit numbers, typically written as four decimal octets separated by periods.

Example:

192.168.1.1

Characteristics

• 32-bit address length
• Approximately 4.3 billion unique addresses
• Uses dotted-decimal notation
• Widely supported by legacy systems
• Requires NAT (Network Address Translation) in many networks due to address exhaustion

Advantages

• Simple and well-established
• Broad compatibility
• Easy to configure and troubleshoot

Limitations

• Limited address space
• Address exhaustion
• Increased reliance on NAT

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IPv6

Definition

Internet Protocol Version 6 (IPv6) was developed to overcome the address limitations of IPv4 and provide improved networking capabilities.

Address Format

IPv6 addresses are 128-bit numbers represented as eight groups of hexadecimal digits separated by colons.

Example:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

Compressed form:

2001:db8:85a3::8a2e:370:7334

Characteristics

• 128-bit address length
• Approximately 340 undecillion unique addresses
• Uses hexadecimal notation
• Supports auto-configuration
• Built-in support for modern networking features

Advantages

• Vast address space
• Improved routing efficiency
• Better support for mobile devices
• Simplified network configuration
• Enhanced security capabilities

Limitations

• Migration complexity
• Not all legacy systems support IPv6
• Requires updates to some network infrastructure

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IPv4 vs IPv6 Comparison

Feature	IPv4	IPv6
Address Length	32 bits	128 bits
Address Format	Decimal	Hexadecimal
Address Example	192.168.1.1	2001:db8::1
Address Space	~4.3 Billion	~340 Undecillion
NAT Required	Often	Usually Not
Auto Configuration	Limited	Built-in
Header Complexity	More Complex	Simplified
Security	Optional IPSec	Native IPSec Support

AWS reserves 5 IPs per subnet (network, router, DNS, future, broadcast).

Rule:

Mask Octet = 256 - 2^(number of host bits)

AWS Subnet Sizing Example

For a VPC with CIDR 10.0.0.0/16 and 4 subnets:

Subnet Name	CIDR Block	AZ	Type	AWS Reserved IPs	Usable IPs
web1 (public)	10.0.1.0/24	ap-south-1a	Public	5	251
web2 (public)	10.0.2.0/24	ap-south-1b	Public	5	251
app1 (private)	10.0.3.0/24	ap-south-1a	Private	5	251
app2 (private)	10.0.4.0/24	ap-south-1b	Private	5	251

AWS VPC Concepts

Key Components

Component	Description
Internet Gateway (IGW)	Allows VPC resources to communicate with the internet
Route Table	Set of rules (routes) that determine where network traffic is directed
Public Subnet	Subnet with a route to the Internet Gateway; instances can have public IPs
Private Subnet	Subnet with no direct route to the internet
Jumpbox / Bastion Host	A machine in the public subnet used to SSH into machines in private subnets

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AWS Regions & Availability Zones (AZs)

Region

An AWS Region is a physical geographic area in the world that contains multiple, isolated data centers (Availability Zones).

| Property | Detail |

Activity — VPC with 4 Subnets

Goal: Create a VPC with 2 public and 2 private subnets, then launch EC2 instances to observe public IP behaviour.

Steps

1. Create a VPC (e.g., CIDR 10.0.0.0/16)
2. Create 4 subnets (2 public, 2 private) across 2 AZs
3. Attach an Internet Gateway to the VPC
4. Create a Route Table for public subnets — add route 0.0.0.0/0 → IGW
5. Associate public subnets with the public route table
6. Launch EC2 instance with public IP in a public subnet → accessible from internet
7. Launch EC2 instance with public IP in a private subnet → not accessible (no IGW route)