Network+ 10-009 Objective 1.7: IPv4 Network Addressing

September 9, 2025•30 min read•CompTIA Network+ Certification

Network+ Exam Focus: This objective covers IPv4 addressing concepts including public vs. private addresses, subnetting techniques, and IPv4 address classes. Understanding IPv4 addressing is fundamental to network design, implementation, and troubleshooting. Master these concepts for both exam success and real-world network administration.

Introduction to IPv4 Addressing

IPv4 (Internet Protocol version 4) addressing is the foundation of network communication. An IPv4 address is a 32-bit number that uniquely identifies devices on a network. Understanding IPv4 addressing concepts is essential for network design, troubleshooting, and management. This guide covers the key aspects of IPv4 addressing that every network professional must master.

IPv4 Address Structure:

32-bit Address: Four octets separated by dots (e.g., 192.168.1.1)
Dotted Decimal Notation: Each octet ranges from 0-255
Network and Host Portions: Divided by subnet mask
Unique Identification: Each device needs a unique IP address
Hierarchical Structure: Network portion identifies the network, host portion identifies the device

Public vs. Private IPv4 Addresses

IPv4 addresses are divided into public and private categories based on their intended use and routing characteristics. Understanding this distinction is crucial for network design and security.

Public IPv4 Addresses

Characteristics:

Globally Unique: Must be unique across the entire internet
Routable: Can be routed across the internet
Assigned by IANA: Internet Assigned Numbers Authority manages allocation
Regional Distribution: Allocated to Regional Internet Registries (RIRs)
Cost: Must be purchased or leased from ISPs
Limited Supply: IPv4 address space is nearly exhausted

Public Address Ranges:

All addresses except private ranges
Examples: 8.8.8.8, 1.1.1.1, 208.67.222.222
Internet Access: Required for direct internet connectivity
Security Considerations: Exposed to internet threats

Private IPv4 Addresses (RFC 1918)

RFC 1918 defines three ranges of private IPv4 addresses that are reserved for use within private networks and are not routable on the public internet.

RFC 1918 Private Address Ranges:

Class A Private: 10.0.0.0 to 10.255.255.255 (10.0.0.0/8)
Class B Private: 172.16.0.0 to 172.31.255.255 (172.16.0.0/12)
Class C Private: 192.168.0.0 to 192.168.255.255 (192.168.0.0/16)

Private Address Characteristics:

Non-Routable: Cannot be routed across the internet
Reusable: Can be used in multiple private networks
Free to Use: No cost for private address ranges
NAT Required: Need Network Address Translation for internet access
Security Benefits: Hidden from direct internet access
Internal Use Only: Designed for private network communication

Private Address Use Cases:

Home Networks: Typically use 192.168.1.0/24 or 192.168.0.0/24
Small Offices: Often use 192.168.x.0/24 ranges
Enterprise Networks: May use 10.0.0.0/8 for large networks
Data Centers: Use private ranges for internal communication
VPN Networks: Private ranges for secure tunnels

Automatic Private IP Addressing (APIPA)

APIPA is a Windows feature that automatically assigns IP addresses to devices when DHCP is unavailable. This ensures devices can still communicate on the local network segment.

APIPA Characteristics:

Address Range: 169.254.0.1 to 169.254.255.254
Subnet Mask: 255.255.0.0 (/16)
Automatic Assignment: No manual configuration required
DHCP Fallback: Used when DHCP server is unavailable
Limited Scope: Only works on local network segment
No Internet Access: Cannot access internet or other networks

APIPA Process:

DHCP Request: Device attempts to obtain IP from DHCP
DHCP Failure: No response from DHCP server
APIPA Assignment: Device assigns itself an APIPA address
Duplicate Detection: Ensures no address conflicts
Periodic Retry: Continues to attempt DHCP renewal

APIPA Troubleshooting:

DHCP Server Issues: Check DHCP server status
Network Connectivity: Verify physical connections
DHCP Scope: Ensure DHCP scope is configured
Firewall Rules: Check for blocked DHCP traffic
Manual Configuration: Assign static IP if needed

Loopback/Localhost

The loopback address is a special IPv4 address used for testing and internal communication on a device. It allows a device to communicate with itself.

Loopback Address Characteristics:

Address Range: 127.0.0.0 to 127.255.255.255
Common Address: 127.0.0.1 (localhost)
Local Only: Never leaves the local device
Testing Purpose: Used for network stack testing
No Routing: Routers do not forward loopback traffic
Always Available: Works even without network interface

Loopback Use Cases:

Network Testing: Test TCP/IP stack functionality
Application Testing: Test applications without network
Service Binding: Bind services to localhost for security
Development: Local development and testing
Troubleshooting: Verify network stack is working

Subnetting

Subnetting is the process of dividing a network into smaller, more manageable subnetworks. This technique allows for better network organization, improved security, and efficient use of IP addresses.

Subnet Mask Basics

Subnet Mask Components:

32-bit Number: Same length as IPv4 address
Binary Format: Series of 1s followed by 0s
Network Bits: 1s identify network portion
Host Bits: 0s identify host portion
Dotted Decimal: Expressed as four octets (e.g., 255.255.255.0)
CIDR Notation: Expressed as /n (e.g., /24)

Common Subnet Masks:

/8 (255.0.0.0): Class A default
/16 (255.255.0.0): Class B default
/24 (255.255.255.0): Class C default
/25 (255.255.255.128): 128 hosts per subnet
/26 (255.255.255.192): 64 hosts per subnet
/27 (255.255.255.224): 32 hosts per subnet
/28 (255.255.255.240): 16 hosts per subnet
/30 (255.255.255.252): 2 hosts per subnet (point-to-point)

Subnetting Calculations

Key Formulas:

Number of Subnets: 2^(subnet bits)
Number of Hosts: 2^(host bits) - 2
Subnet Increment: 256 - last octet of subnet mask
Block Size: 2^(host bits)

Subnetting Example (192.168.1.0/26):

Subnet Mask: 255.255.255.192
Host Bits: 6 bits
Number of Hosts: 2^6 - 2 = 62 hosts per subnet
Subnet Increment: 256 - 192 = 64
Subnets: 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/26

Variable Length Subnet Mask (VLSM)

VLSM allows different subnets within the same network to have different subnet masks, enabling more efficient use of IP address space.

VLSM Characteristics:

Different Mask Lengths: Subnets can have different sizes
Efficient Allocation: Right-size subnets for actual needs
Address Conservation: Reduces wasted IP addresses
Hierarchical Design: Supports complex network designs
Routing Support: Requires routing protocols that support VLSM
Complex Management: More complex than fixed-length subnetting

VLSM Example (192.168.1.0/24):

LAN 1 (50 hosts): 192.168.1.0/26 (62 hosts)
LAN 2 (25 hosts): 192.168.1.64/27 (30 hosts)
LAN 3 (10 hosts): 192.168.1.96/28 (14 hosts)
Point-to-Point: 192.168.1.112/30 (2 hosts)
Remaining: 192.168.1.116/30 to 192.168.1.252/30

Classless Inter-Domain Routing (CIDR)

CIDR is a method for allocating IP addresses and routing IP packets that allows for more flexible address allocation than the traditional classful addressing system.

CIDR Characteristics:

Prefix Notation: Uses /n notation (e.g., /24)
Classless: Not limited by traditional class boundaries
Route Aggregation: Allows multiple routes to be summarized
Flexible Allocation: Any number of bits for network portion
Efficient Routing: Reduces routing table size
Modern Standard: Used in modern IP networks

CIDR Examples:

192.168.1.0/24: 256 addresses (192.168.1.0 - 192.168.1.255)
10.0.0.0/8: 16,777,216 addresses (10.0.0.0 - 10.255.255.255)
172.16.0.0/12: 1,048,576 addresses (172.16.0.0 - 172.31.255.255)
203.0.113.0/25: 128 addresses (203.0.113.0 - 203.0.113.127)

IPv4 Address Classes

IPv4 addresses were originally divided into five classes (A, B, C, D, E) based on the first few bits of the address. While classful addressing is largely obsolete, understanding these classes is still important for network education and legacy systems.

Class A Addresses

Characteristics:

Range: 1.0.0.0 to 126.255.255.255
First Bit: Always 0 (binary: 0xxxxxxx)
Network Bits: 8 bits (first octet)
Host Bits: 24 bits (last three octets)
Default Mask: 255.0.0.0 (/8)
Number of Networks: 126 (excluding 0 and 127)
Hosts per Network: 16,777,214

Class A Examples:

10.0.0.0/8: Private Class A range
1.1.1.1: Public Class A address
8.8.8.8: Google DNS (Class A)
Use Cases: Large organizations, ISPs

Class B Addresses

Characteristics:

Range: 128.0.0.0 to 191.255.255.255
First Two Bits: Always 10 (binary: 10xxxxxx)
Network Bits: 16 bits (first two octets)
Host Bits: 16 bits (last two octets)
Default Mask: 255.255.0.0 (/16)
Number of Networks: 16,384
Hosts per Network: 65,534

Class B Examples:

172.16.0.0/12: Private Class B range
172.16.1.0/24: Private Class B subnet
Use Cases: Medium to large organizations

Class C Addresses

Characteristics:

Range: 192.0.0.0 to 223.255.255.255
First Three Bits: Always 110 (binary: 110xxxxx)
Network Bits: 24 bits (first three octets)
Host Bits: 8 bits (last octet)
Default Mask: 255.255.255.0 (/24)
Number of Networks: 2,097,152
Hosts per Network: 254

Class C Examples:

192.168.0.0/16: Private Class C range
192.168.1.0/24: Common home network
Use Cases: Small networks, home networks

Class D Addresses

Characteristics:

Range: 224.0.0.0 to 239.255.255.255
First Four Bits: Always 1110 (binary: 1110xxxx)
Purpose: Multicast addresses
Not for Hosts: Cannot be assigned to individual devices
Multicast Groups: Used for group communication
Special Addresses: Some reserved for specific protocols

Class D Examples:

224.0.0.1: All Systems multicast
224.0.0.2: All Routers multicast
224.0.0.22: IGMP multicast
239.0.0.0/8: Private multicast range

Class E Addresses

Characteristics:

Range: 240.0.0.0 to 255.255.255.255
First Four Bits: Always 1111 (binary: 1111xxxx)
Purpose: Reserved for experimental use
Not Routable: Cannot be used on public internet
Research Only: Reserved for future use
Special Addresses: 255.255.255.255 is broadcast

Address Class Summary Table

Class	Range	Default Mask	Networks	Hosts/Network	Purpose
A	1.0.0.0 - 126.255.255.255	/8	126	16,777,214	Large networks
B	128.0.0.0 - 191.255.255.255	/16	16,384	65,534	Medium networks
C	192.0.0.0 - 223.255.255.255	/24	2,097,152	254	Small networks
D	224.0.0.0 - 239.255.255.255	N/A	N/A	N/A	Multicast
E	240.0.0.0 - 255.255.255.255	N/A	N/A	N/A	Reserved

Special IPv4 Addresses

Reserved and Special Addresses:

0.0.0.0: Default route, "this network"
127.0.0.0/8: Loopback addresses
169.254.0.0/16: APIPA addresses
224.0.0.0/4: Multicast addresses
240.0.0.0/4: Reserved addresses
255.255.255.255: Limited broadcast

Subnetting Practice Examples

Example 1: Basic Subnetting

Given: 192.168.1.0/24, need 4 subnets

Subnet Bits: Need 2 bits (2^2 = 4 subnets)
New Mask: /26 (255.255.255.192)
Host Bits: 6 bits (2^6 - 2 = 62 hosts per subnet)
Subnets:
- 192.168.1.0/26 (192.168.1.1 - 192.168.1.62)
- 192.168.1.64/26 (192.168.1.65 - 192.168.1.126)
- 192.168.1.128/26 (192.168.1.129 - 192.168.1.190)
- 192.168.1.192/26 (192.168.1.193 - 192.168.1.254)

Example 2: VLSM Subnetting

Given: 10.0.0.0/8, need subnets for different sizes

LAN A (1000 hosts): 10.0.0.0/22 (1022 hosts)
LAN B (500 hosts): 10.0.4.0/23 (510 hosts)
LAN C (100 hosts): 10.0.6.0/25 (126 hosts)
Point-to-Point: 10.0.6.128/30 (2 hosts)

Common Exam Scenarios

Network+ exam questions often test your understanding of IPv4 addressing in practical scenarios. Here are common topics:

Scenario-Based Questions:

Subnetting Calculations: Determine subnet masks and host ranges
Address Classification: Identify address classes and types
Private vs. Public: Distinguish between address types
VLSM Design: Create efficient subnet allocations
Troubleshooting: Identify addressing problems

Study Tips for Network+ Objective 1.7

Key Study Points:

Memorize Private Ranges: Know RFC 1918 addresses by heart
Practice Subnetting: Work through many subnetting examples
Understand Classes: Know the characteristics of each address class
Special Addresses: Remember loopback, APIPA, and broadcast addresses
CIDR Notation: Be comfortable with /n notation
Binary Conversion: Practice converting between binary and decimal

Conclusion

IPv4 addressing is a fundamental concept that every network professional must master. Understanding the differences between public and private addresses, the principles of subnetting, and the characteristics of different address classes is essential for network design, implementation, and troubleshooting.

While classful addressing is largely obsolete in modern networks, the concepts still provide valuable insight into IP addressing principles. CIDR and VLSM have made IP addressing more flexible and efficient, allowing for better utilization of the limited IPv4 address space.

Next Steps: Practice subnetting calculations regularly and work through various scenarios. Understanding IPv4 addressing concepts will help you design efficient networks and troubleshoot connectivity issues effectively.