Chapter 4: IP Addressing Basics
Understanding IPv4, IPv6, subnetting, and address resolution

Introduction
IP addressing is the foundation of network communication, enabling devices to identify and locate each other. This chapter explores both IPv4 and IPv6 addressing schemes, subnetting techniques, and how devices resolve IP addresses to physical MAC addresses.
By the end of this chapter, you will be able to:
- Compare IPv4 and IPv6 address structures
- Perform subnetting and supernetting calculations
- Differentiate between private and public IP addresses
- Explain CIDR notation and its benefits
- Describe the ARP process and its role in networking
IPv4 vs IPv6: Structure and Evolution
IPv4 (Internet Protocol version 4)
- Address Size: 32 bits (4.3 billion addresses)
- Format: Four 8-bit octets (0-255) in dotted decimal
- Limitations: Address exhaustion, NAT dependency
- Header: 20+ bytes with options, checksum
- Example: 192.168.1.1 with subnet mask 255.255.255.0
IPv6 (Internet Protocol version 6)
- Address Size: 128 bits (340 undecillion addresses)
- Format: Eight 16-bit segments in hexadecimal
- Advantages: No NAT needed, built-in security
- Header: Fixed 40 bytes, no checksum
- Example: 2001:db8::1 (compressed format)
Figure 1: Structural comparison of IPv4 and IPv6 addresses
IPv6 Transition Mechanisms
Dual Stack
Devices run both IPv4 and IPv6 simultaneously
Tunneling
Encapsulate IPv6 packets in IPv4 for traversal
Translation
NAT64/DNS64 convert between protocols
Common Pitfall
When configuring IPv6, remember that interface identifiers (last 64 bits) can be derived from MAC addresses (EUI-64 format), potentially exposing device information. Use privacy extensions when needed.
Subnetting: Dividing Networks
Subnetting breaks large networks into smaller, manageable subnets for improved performance and security. Let's subnet 192.168.1.0/24 into 4 subnets:
Step 1: Determine Subnet Requirements
- Original network: 192.168.1.0/24 (256 addresses)
- Required subnets: 4
- Hosts per subnet: ~60 (62 usable)
Step 2: Calculate New Subnet Mask
- Borrow 2 bits (2²=4 subnets)
- New prefix: /26 (255.255.255.192)
- Host bits: 6 (2⁶-2=62 hosts per subnet)
Step 3: Subnet Ranges
Subnet | Network Address | First Host | Last Host | Broadcast |
---|---|---|---|---|
1 | 192.168.1.0/26 | 192.168.1.1 | 192.168.1.62 | 192.168.1.63 |
2 | 192.168.1.64/26 | 192.168.1.65 | 192.168.1.126 | 192.168.1.127 |
3 | 192.168.1.128/26 | 192.168.1.129 | 192.168.1.190 | 192.168.1.191 |
4 | 192.168.1.192/26 | 192.168.1.193 | 192.168.1.254 | 192.168.1.255 |
Figure 2: Visualization of a /24 network divided into four /26 subnets
Subnetting Cheat Sheet
Prefix | Mask | Subnets | Hosts | Block Size |
---|---|---|---|---|
/24 | 255.255.255.0 | 1 | 254 | 256 |
/25 | 255.255.255.128 | 2 | 126 | 128 |
/26 | 255.255.255.192 | 4 | 62 | 64 |
/27 | 255.255.255.224 | 8 | 30 | 32 |
/28 | 255.255.255.240 | 16 | 14 | 16 |
Supernetting: Aggregating Routes
Supernetting combines contiguous networks into larger blocks to reduce routing table size. Consider these four /24 networks:
Step 1: Check Contiguity
All networks are contiguous in the third octet (0-3).
Step 2: Find Common Bits
192.168.0.0: 11000000.10101000.00000000.00000000
192.168.1.0: 11000000.10101000.00000001.00000000
192.168.2.0: 11000000.10101000.00000010.00000000
192.168.3.0: 11000000.10101000.00000011.00000000
First 22 bits are identical → /22 supernet
Resulting Supernet
Troubleshooting Tip
When supernetting, ensure all networks are truly contiguous. Gaps (like missing 192.168.1.0/24) would require multiple routing entries.
IP Address Classes and Private vs. Public IPs
IPv4 Address Classes (Historical)
Class | Range | Network/Host | Purpose |
---|---|---|---|
A | 1.0.0.0 - 126.255.255.255 | N.H.H.H | Large networks |
B | 128.0.0.0 - 191.255.255.255 | N.N.H.H | Medium networks |
C | 192.0.0.0 - 223.255.255.255 | N.N.N.H | Small networks |
D | 224.0.0.0 - 239.255.255.255 | - | Multicast |
E | 240.0.0.0 - 255.255.255.255 | - | Experimental |
Private vs. Public IP Addresses
Private IP Ranges (RFC 1918)
- 10.0.0.0/8 (16.7 million addresses)
- 172.16.0.0/12 (1 million addresses)
- 192.168.0.0/16 (65,536 addresses)
- Not routable on public Internet
- Used with NAT for Internet access
Public IP Ranges
- All other IPv4 addresses
- Globally routable on Internet
- Assigned by IANA to RIRs
- Must be unique worldwide
- Example: Your web server's IP
CIDR: Modern IP Allocation
Classless Inter-Domain Routing (CIDR) replaced classful addressing with flexible network sizes:
CIDR Notation
- Slash notation (/24, /26, etc.)
- Prefix indicates network bits
- More efficient than fixed classes
Benefits
- Reduced routing table size
- Flexible network sizes
- Better address space utilization
- Supports VLSM (Variable Length Subnet Masking)
CIDR Allocation Example
An ISP might receive 203.0.113.0/24 from their RIR, then allocate:
- 203.0.113.0/26 to Customer A (62 hosts)
- 203.0.113.64/27 to Customer B (30 hosts)
- 203.0.113.96/28 to Customer C (14 hosts)
- 203.0.113.112/28 reserved for future use
Address Resolution Protocol (ARP)
ARP maps IP addresses to MAC addresses on local networks:
Figure 3: ARP request/reply process between devices on a local network
ARP Process
- ARP Request: Broadcast "Who has 192.168.1.2?"
- ARP Reply: Unicast "192.168.1.2 is at 00:1A:2B:3C:4D:5E"
- Cache Update: Both devices store mapping in ARP table
- Communication: Frames sent with correct MAC addresses
Viewing ARP Cache
Windows
arp -a
Interface: 192.168.1.10
Internet Address Physical Address Type
192.168.1.1 00-1a-2b-3c-4d-5e dynamic
192.168.1.255 ff-ff-ff-ff-ff-ff static
Linux/macOS
arp -an
? (192.168.1.1) at 00:1a:2b:3c:4d:5e [ether] on eth0
Security Consideration
ARP spoofing attacks can redirect traffic. Mitigate with:
- Static ARP entries for critical devices
- Port security on switches
- ARP inspection tools
Practical Example: Configuring IP Addresses in Packet Tracer
Let's configure a router interface and two PCs with IP addresses from our subnetting example:
Step 1: Router Configuration
Router> enable
Router# configure terminal
Router(config)# interface gigabitEthernet 0/0
Router(config-if)# ip address 192.168.1.1 255.255.255.192
Router(config-if)# no shutdown
Router(config-if)# exit
Step 2: PC1 Configuration
IP Address: 192.168.1.2
Subnet Mask: 255.255.255.192
Default Gateway: 192.168.1.1
Step 3: PC2 Configuration
IP Address: 192.168.1.3
Subnet Mask: 255.255.255.192
Default Gateway: 192.168.1.1
Step 4: Verify Connectivity
PC1> ping 192.168.1.3
Pinging 192.168.1.3 with 32 bytes of data:
Reply from 192.168.1.3: bytes=32 time=1ms TTL=128
Visualizing Subnet Communication
Figure 4: Interactive visualization of devices communicating across subnets
How This Visualization Works
The diagram shows:
- Two subnets (192.168.1.0/26 and 192.168.1.64/26)
- A router connecting the subnets
- ARP resolution within each subnet
- Routing between subnets
Chapter Summary
Key Concepts
- IPv4 uses 32-bit addresses while IPv6 uses 128-bit hexadecimal addresses
- Subnetting divides networks; supernetting combines them for routing efficiency
- Private IP ranges (RFC 1918) are for internal networks only
- CIDR enables flexible network sizes and efficient address allocation
- ARP resolves IP addresses to MAC addresses on local networks
Best Practices
- Plan subnet sizes carefully to allow for growth
- Use private IPs for internal devices and NAT for Internet access
- Document your IP allocation scheme thoroughly
- Monitor ARP tables for suspicious entries
- Begin IPv6 adoption planning even if primarily using IPv4
Further Reading
- RFCs: RFC 791 (IPv4), RFC 2460 (IPv6), RFC 1918 (Private Addressing), RFC 826 (ARP)
- Books: "TCP/IP Illustrated, Volume 1" by W. Richard Stevens
- Tools: Subnet calculators, Wireshark for ARP analysis
- Practice: Cisco Packet Tracer subnetting labs