In computer networking, every device that communicates over a network requires an address. The Internet Protocol version 4 (IPv4) defines a system of logical addresses that enables end-to-end communication across interconnected networks. Unlike physical addresses (MAC addresses), which are burned into hardware and operate only within a single local network segment, IPv4 logical addresses are assigned by software and are hierarchically structured to support routing across the global internet .

Understanding IPv4 addressing is foundational to networking. This lesson covers the structure of IPv4 addresses, the relationship between binary and dotted-decimal notation, how subnet masks separate network and host portions, the categories of special-use address ranges, and how to determine whether two hosts reside on the same network.

The diagram above illustrates the TCP/IP protocol stack. The Network Layer (Layer 3) is where IPv4 logical addressing operates, enabling packets to be routed between networks. The Data Link Layer (Layer 2) uses physical MAC addresses for local frame delivery within a single broadcast domain .

Footnotes

1. Internet Logical Addressing - Explains how logical addresses work globally across the internet while physical addresses operate locally within a LAN. ↩ ↩²

Logical Addressing vs. Physical Addressing

A key distinction in networking is between logical and physical addresses. Both are essential, but they serve fundamentally different roles in the communication process:

The logical address is what allows a router to make forwarding decisions. When a packet crosses from one network to another, the source and destination MAC addresses change at each hop, but the source and destination IPv4 addresses remain constant throughout the journey. This is the fundamental reason logical addressing exists: to provide a persistent, routable identity for end-to-end delivery 2.

Footnotes

1. Internet Logical Addressing - Explains how logical addresses work globally across the internet while physical addresses operate locally within a LAN. ↩

2. IPv4 Address Structure - peachyPing - Detailed breakdown of IPv4 dotted-decimal notation, octets, binary conversion, and the network/host division determined by subnet masks. ↩

IPv4 Address Structure and Notation

An IPv4 address is a 32-bit binary number. For human readability, it is expressed in dotted-decimal notation, which divides the 32 bits into four groups of 8 bits. Each 8-bit group is called an octet .

Example:

The IPv4 address 192.168.1.65 in binary is:

$192.168.1.65 \rightarrow 11000000.10101000.00000001.01000001$

Each octet is converted independently:

• $192 = 128 + 64 = 11000000_2$
• $168 = 128 + 32 + 8 = 10101000_2$
• $1 = 00000001_2$
• $65 = 64 + 1 = 01000001_2$

Since each octet is 8 bits, the minimum value is $00000000_2 = 0$ and the maximum is $11111111_2 = 255$. Therefore, every octet in dotted-decimal notation must fall within the range $[0, 255]$ .

The total IPv4 address space contains $2^{32} = 4{,}294{,}967{,}296$ unique addresses.

Footnotes

1. IPv4 Address Structure - peachyPing - Detailed breakdown of IPv4 dotted-decimal notation, octets, binary conversion, and the network/host division determined by subnet masks. ↩ ↩²

Network and Host Portions: The Role of the Subnet Mask

Every IPv4 address is logically divided into two parts:

1. Network Portion — Identifies the specific network (like a street name).
2. Host Portion — Identifies a specific device on that network (like a house number).

The subnet mask determines where this boundary falls. A subnet mask is a contiguous string of 1-bits (marking the network portion) followed by a contiguous string of 0-bits (marking the host portion) 2.

Example with 192.168.1.65 and mask 255.255.255.0 (/24):

The network address is obtained by performing a bitwise AND between the IP address and the subnet mask. The /24 in CIDR notation indicates that the first 24 bits are the network portion 2.

Footnotes

1. TCP/IP Addressing and Subnetting - Microsoft Learn - Comprehensive guide on how subnet masks divide IP addresses and how the TCP/IP stack determines local vs. remote hosts. ↩ ↩²

2. Subnet - Wikipedia - Defines subnetting, CIDR notation, the bitwise AND operation for network prefix determination, and subnet host count formulas. ↩ ↩²

Public, Private, and Special-Use IPv4 Address Ranges

Not all IPv4 addresses are routable on the public internet. The Internet Assigned Numbers Authority (IANA) and the IETF have reserved specific ranges for private networks, loopback testing, link-local auto-configuration, and multicast communication 2.

Private Address Ranges (RFC 1918)

These addresses are used within internal networks and are not routable on the public internet. Network Address Translation (NAT) is used at the network boundary to map private addresses to public ones.

Other Special-Use Ranges

Public addresses are all remaining IPv4 addresses that are globally routable on the internet, assigned by Regional Internet Registries (RIRs) such as ARIN, RIPE NCC, and APNIC .

Footnotes

1. IPv4 Private Address Space and Filtering - ARIN - Official ARIN reference on RFC 1918 private address ranges and filtering considerations for network operators. ↩ ↩²

2. TCP/IP Addressing and Subnetting - Microsoft Learn - Comprehensive guide on how subnet masks divide IP addresses and how the TCP/IP stack determines local vs. remote hosts. ↩