CCNA / Networking Fundamentals

IP Address & Subnetting

Your complete interactive guide to understanding IP addressing, subnet masks, CIDR notation, and subnetting from scratch.

32

Bits in IPv4

4.3B

Total IPs (2³²)

5

IP Classes

/0–/32

CIDR Range

IP Address Fundamentals

What is an IP Address?

An IPv4 address is a 32-bit number that uniquely identifies every device on a network. It's written in dotted decimal notation — four numbers (0–255) separated by dots.

156 . 89 . 24 . 15

Each group is called an octet (8 bits). Four octets × 8 bits = 32 bits total.

Two Parts of Every IP

Network Bits

Same for ALL devices on the same LAN. Identifies the network. Cannot be changed.

Host Bits

Unique for EACH device on the network. Identifies the specific host. Can be changed.

Anatomy of an IP Address — 158.80.164.3

158

10011110

1st Octet

80

01010000

2nd Octet

164

10100100

3rd Octet

3

00000011

4th Octet

■ Network portion

■ Host portion (Class C example)

Range: 0 per octet (00000000₂) → 255 per octet (11111111₂)

Interactive — IP to Binary Converter

Total IPv4 Addresses

2³² = 4,294,967,296 ≈ 4.3 Billion unique addresses worldwide.

Octet Range

Min: 0 (00000000₂) — Max: 255 (11111111₂) per octet.

Default Gateway

0.0.0.0 is used for default routing and as a wildcard mask / DHCP placeholder.

Loopback Address

127.0.0.0 – 127.255.255.255 — reserved for NIC/TCP-IP testing. Your own machine.

Network ID

First IP in a subnet. All host bits are turned OFF (0). Identifies the network on a router.

Broadcast ID

Last IP in a subnet. All host bits are turned ON (1). Sends to every device on the LAN.

IP Address Classes

Class Range — First Octet (0 to 255)

Class A
0–127

Class B
128–191

Class C
192–223

D
224–239

E
240–255

Class Details & Specifications

Class	First Octet	Default Mask	Network Bits	Host Bits	Max Hosts/Net	Purpose
A	1 – 126	255.0.0.0 /8	8 (N.H.H.H)	24	16,777,214	ISPs, large orgs
B	128 – 191	255.255.0.0 /16	16 (N.N.H.H)	16	65,534	Medium networks
C	192 – 223	255.255.255.0 /24	24 (N.N.N.H)	8	254	Small/home nets
D	224 – 239	—	—	—	—	Multicast only
E	240 – 255	—	—	—	—	Experimental/Research

Class A Special

0.0.0.0 — Default routing / DHCP
127.x.x.x — Loopback (NIC test)
10.0.0.0/8 — Private IP range
Usable public: 1–9 and 11–126

Class B Special

APIPA: 169.254.0.0/16
↳ Auto Private IP (no DHCP)
Private: 172.16.0.0 – 172.31.255.255 /12

Class C Special

Private: 192.168.0.0 – 192.168.255.255
Subnet: /24 (255.255.255.0)
Most common in home/office networks

Private vs Public IP

Attribute	🔒 Private IP	🌐 Public IP
Assigned by	Network Administrator	ISP (Internet Service Provider)
Uniqueness	Locally unique (within LAN)	Globally unique (worldwide)
Registration	Unregistered	Registered
Cost	Free	Paid
Used for	LAN / Internal communication	WAN / Internet communication
Ranges	A 10.0.0.0 – 10.255.255.255 B 172.16.0.0 – 172.31.255.255 C 192.168.0.0 – 192.168.255.255	All IPs not listed as private. Routable on the internet.

💡 Memory Tip

Private IPs are like your home address within a building complex — unique inside, but unknown to the outside world. Public IPs are like the building's street address — visible to everyone globally.

Subnet Mask

What is a Subnet Mask?

A subnet mask is a 32-bit address written in dotted decimal, just like an IP. It tells you which part of an IP address is the network and which part is the host.

🔵 ON bits (1s) → define the network portion
🟠 OFF bits (0s) → define the host portion

Default Subnet Masks by Class

Class A255.0.0.0N.H.H.H/8

Class B255.255.0.0N.N.H.H/16

Class C255.255.255.0N.N.N.H/24

Classful vs Classless

Classful (Default)

Uses the default subnet mask for the IP class. Simple but wastes many IP addresses.

192.168.1.0 / 255.255.255.0 (/24)

Classless (CIDR)

Uses any subnet mask with any class. Prevents IP waste. Used in modern networks.

192.168.1.0 / 255.255.255.192 (/26)

CIDR Notation

What is CIDR?

Classless Inter-Domain Routing (CIDR) is a compact way to write subnet masks by counting the number of consecutive ON bits (1s) from the left.

Example: 255.255.248.0 in binary is:

■ 21 ON bits = /21■ 11 OFF bits = host portionSo: 255.255.248.0 = /21

Block Size Reference — 4th Octet

CIDR	Subnet Mask (4th octet)	Borrowed Bits	Subnets (2ⁿ)	Block Size	Hosts/Subnet (2ʰ−2)	Last ON bit
/24	.0	0	1	256	254	—
/25	.128	1	2	128	126	128
/26	.192	2	4	64	62	64
/27	.224	3	8	32	30	32
/28	.240	4	16	16	14	16
/29	.248	5	32	8	6	8
/30	.252	6	64	4	2	4
/31	.254	7	128	2	0 (P2P only)	2
/32	.255	8	256	1	0 (host route)	1

📐 Block Size Rule

Block Size = value of the last ON bit in the subnet mask. Example: /26 → last ON bit is at position 64 → block size = 64. This is also = 256 − the 4th octet value of the subnet mask.

Subnetting Deep Dive

What is Subnetting?

Subnetting is the process of dividing a large network into multiple smaller sub-networks (subnets). This is done by borrowing bits from the host portion of an IP address and using them for additional network bits.

⚡

Better Performance

Reduces broadcast traffic

💎

Efficient IPs

Minimizes IP waste

🔐

Better Security

Isolates network segments

🎛️

More Control

Easier management

Subnetting Formulas

2ⁿ

Number of Subnets
n = number of borrowed bits (extra ON bits added to the default mask)

2ʰ − 2

Hosts per Subnet
h = number of remaining host bits (OFF bits in the mask). −2 removes Network ID and Broadcast ID.

Last ON bit

Block Size
The value of the last set bit (1) in the subnet mask. Each subnet starts at multiples of this block size.

Worked Example — 192.168.10.0 /26

Starting from a Class C default /24, we borrow 2 bits making it /26.

Bit positions: 128 64 | 32 16 8 4 2 1
Borrowed: 1 1 | 0 0 0 0 0 0
↑ ↑
n=2 borrowed → 2² = 4 subnets
h=6 host bits → 2⁶−2 = 62 hosts
Block size → 64 (last ON bit)

0–63

64–127

128–191

192–255

#	Network ID	First Host	Last Host	Broadcast
1	.0	.1	.62	.63
2	.64	.65	.126	.127
3	.128	.129	.190	.191
4	.192	.193	.254	.255

Subnet Mask: 255.255.255.192

Interactive Subnet Calculator

Network IP Address

CIDR Prefix (e.g. /26)

4

Subnets (2^2)

62

Hosts/Subnet

255.255.255.192

Subnet Mask

64

Block Size

2

Borrowed Bits

6

Host Bits

Showing 4 of 4 subnets:

#
Network ID
First–Last Host
Broadcast

1
168.1.0
168.1.1 – 192.168.1.62
168.1.63

2
168.1.64
168.1.65 – 192.168.1.126
168.1.127

3
168.1.128
168.1.129 – 192.168.1.190
168.1.191

4
168.1.192
168.1.193 – 192.168.1.254
168.1.255

FLSM vs VLSM

FLSM — Fixed Length Subnet Mask

All subnets are thesame size
Uses default /8, /16, /24 masks per class
Simple to design and implement
Wastes IP addresses (not ideal for modern use)
Good for study/learning purposes

FLSM EXAMPLE

192.168.1.0 /26 → 4 subnets all with 62 hosts each
Even if one subnet only needs 5 hosts — wastes 57!

VLSM — Variable Length Subnet Mask

Subnets can bedifferent sizes
Uses any subnet mask with any class
Efficient use of IP address space
Used in all real-world networks today
Enables route summarization

VLSM EXAMPLE

168.1.0 /26 → 62 hosts (large dept)
168.1.64 /28 → 14 hosts (small dept)
168.1.80 /30 → 2 hosts (P2P link)

🌐 Real World

VLSM is what every real network engineer uses. For example, router-to-router (point-to-point) links use /30 (only 2 hosts needed), while a large office uses /22 (1022 hosts). VLSM ensures zero waste.

Route Summarization (Supernetting)

What is Route Summarization?

Route Summarization (also called Supernetting or Aggregation) combines multiple smaller subnets into one larger summary route. This reduces the size of routing tables and improves router efficiency.

Step-by-Step Example

Summarize: 192.168.100.0/24 through 192.168.110.0/24

1

Identify the Range

Find the lowest and highest network addresses in the group.

Lowest: 192.168.100.0
Highest: 192.168.110.0

2

Convert the Differing Octet to Binary

Focus on the 3rd octet (where networks differ).

100 = 0110 0100
110 = 0110 1110

3

Find the Common Bits (Left to Right)

Compare bit-by-bit from the left. Stop when bits first differ.

100 = 0110 0100
110 = 0110 1110
~~~~
Common: 0110 0 = first 5 bits match → but we check crossing over 11 networks
→ 01100000 = 96 (set differing bits to 0)

4

Count Common Bits for CIDR

Total common bits across octets 1+2+3 gives the CIDR prefix.

Octet 1: 8 common bits (192 = same)
Octet 2: 8 common bits (168 = same)
Octet 3: 3 common bits (0110 0...)
Total: 8 + 8 + 3 = /19

✓

Result — Summary Network

Summary Route: 192.168.96.0 /19
Subnet Mask: 255.255.224.0
Covers: 192.168.96.0 → 192.168.127.255

🚀 Why Route Summarization Matters

Instead of advertising 11 individual /24 routes to all routers, you advertise just ONE /19 summary route. This keeps routing tables small, reduces memory usage on routers, and speeds up routing decisions across large networks.

Quick Reference

Network ID

First IP of a subnet. All host bits = 0. Used to identify the network on routers globally.

Broadcast ID

Last IP of a subnet. All host bits = 1. Sends packet to ALL devices on the LAN.

Loopback

127.0.0.1 — Points back to your own device. Used to test the TCP/IP stack without needing a network.

APIPA

169.254.x.x — Auto-assigned when DHCP fails. Means your device can't reach the DHCP server.

CIDR

Shorthand for subnet masks — /24 = 255.255.255.0. Counts consecutive ON bits from the left.

Borrowed Bits

Extra bits taken from the host portion to create more subnets. Each borrowed bit doubles the subnet count.

Block Size

Value of the last ON bit in subnet mask. Each subnet starts at multiples of this value. /26 → 64.

Supernetting

Combining multiple subnets into one summary route. Reduces routing table entries. Opposite of subnetting.

Multicast

Class D (224–239) IPs send one packet to a group of devices subscribed to a multicast group.

The 3 Golden Formulas

2ⁿ

Number of Subnets
n = borrowed bits

2ʰ−2

Number of Hosts
h = host bits left

256−mask

Block Size
256 − last mask octet

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Tech With Your Friend

CCNA Notes 3 # IP Address & Subnetting Explained — Complete Visual Guide for Beginners (CCNA)

IP Address & Subnetting

IP Address Fundamentals

IP Address Classes

Private vs Public IP

Subnet Mask

Classful (Default)

Classless (CIDR)

CIDR Notation

Subnetting Deep Dive

FLSM vs VLSM

FLSM — Fixed Length Subnet Mask

VLSM — Variable Length Subnet Mask

Route Summarization (Supernetting)

Quick Reference

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