An IP address is a numerical address assigned to a network interface so devices can identify sources and destinations when communicating over IP networks.

Just as your home needs a street address to receive mail, computers, smartphones, servers, and other networked devices need IP addresses to send and receive data across networks. IPv4 (Internet Protocol version 4) has been the dominant addressing system powering internet communications since the 1980s.

This guide explains what IPv4 addresses are, how they’re structured, the different types and classes, and how they enable devices to communicate across networks. Understanding IPv4 basics provides the foundation for understanding modern networking concepts and the eventual transition to IPv6.

What Is IPv4?

IPv4 stands for Internet Protocol version 4, the fourth iteration of the Internet Protocol and the first version to be widely deployed for public internet use. It defines the rules and format for addressing devices on networks, enabling data to travel from source to destination across interconnected networks.

Developed in the early 1980s, IPv4 uses a 32-bit addressing scheme that provides approximately 4.3 billion unique addresses. While this seemed more than adequate when designed for early academic and military networks, the explosive growth of internet-connected devices has created address scarcity that drives the need for IPv6.

IPv4 remains widely deployed across the internet, and many networks support IPv4 and IPv6 in parallel. Its mature infrastructure and broad compatibility mean IPv4 will continue to coexist with IPv6 for the foreseeable future.

Pro Tip: When people refer to “IP addresses” in general conversation, they typically mean IPv4 addresses. IPv6 addresses are usually specified explicitly due to their different format and longer length.

IPv4 Address Structure and Format

IPv4 addresses consist of four numbers separated by periods (dots), with each number ranging from 0 to 255. This format is called dotted decimal notation.

Examples of valid IPv4 addresses include 192.168.1.1 (common home router address), 8.8.8.8 (Google’s public DNS server), 172.16.0.1 (private network address), and 203.0.113.45 (documentation-only example address).

Binary Representation

Behind the human-readable decimal format, IPv4 addresses are actually 32-bit binary numbers. Each of the four decimal numbers (called octets) represents 8 bits.

The address 192.168.1.1 in binary is: 11000000.10101000.00000001.00000001

Each octet can represent values from 0 (00000000 in binary) to 255 (11111111 in binary), which is why IPv4 addresses always use numbers in this range.

This 32-bit structure provides 2^32 possible addresses, equaling 4,294,967,296 total unique IPv4 addresses—the theoretical maximum before address reuse through NAT (Network Address Translation) or other techniques.

Network and Host Portions

Every IPv4 address contains two parts: the network portion identifying which network the device belongs to, and the host portion identifying the specific device within that network.

The division between network and host portions depends on the subnet mask, which determines how many bits represent the network versus the host. This division allows efficient routing because routers only need to examine the network portion to determine where to send packets.

For example, in the address 192.168.1.100 with subnet mask 255.255.255.0, the first three octets (192.168.1) identify the network, while the last octet (100) identifies the specific host on that network.

IPv4 Address Classes (Historical Overview)

IPv4 addresses were originally divided into five classes (A, B, C, D, E) based on their intended use and network size. While Classless Inter-Domain Routing (CIDR) has largely replaced this system, understanding address classes remains relevant for networking fundamentals.

Class A Addresses

Class A addresses range from 1.0.0.0 to 126.255.255.255. The first octet identifies the network, while the remaining three octets identify hosts.

Class A networks support approximately 16 million hosts per network, making them suitable for very large organizations. Few Class A networks exist because only 126 are possible (network addresses 1 through 126).

The first bit of Class A addresses is always 0 in binary, which is how routers originally identified the class.

Class B Addresses

Class B addresses range from 128.0.0.0 to 191.255.255.255. The first two octets identify the network, while the last two identify hosts.

Class B networks support approximately 65,000 hosts per network, suitable for medium to large organizations. There are about 16,000 possible Class B networks.

The first two bits of Class B addresses are always 10 in binary.

Class C Addresses

Class C addresses range from 192.0.0.0 to 223.255.255.255. The first three octets identify the network, while only the last octet identifies hosts.

Class C networks support 254 hosts per network (256 minus network and broadcast addresses), suitable for small businesses and home networks. Over 2 million Class C networks are possible.

The first three bits of Class C addresses are always 110 in binary.

Class D Multicast and Class E Reserved Addresses

Class D addresses (224.0.0.0 to 239.255.255.255) are reserved for multicast groups, allowing efficient one-to-many communication where one sender transmits data simultaneously to multiple receivers.

The range historically known as Class E, beginning at 240.0.0.0, is reserved rather than available for ordinary host addressing. The address 255.255.255.255 has a separate special purpose as the limited broadcast address.

Pro Tip: Modern networks use CIDR notation (like 192.168.1.0/24) instead of address classes for more flexible and efficient address allocation. The /24 indicates how many bits represent the network portion, replacing the rigid class system.

Public vs Private IPv4 Addresses

IPv4 addresses are divided into public addresses routable on the internet and private addresses used only within local networks.

Public IP Addresses

Public IP addresses are globally unique addresses assigned by Internet Service Providers (ISPs) and routing authorities. These addresses are accessible from anywhere on the internet.

When you visit a website, your device uses its public IP address to communicate with the web server’s public IP address. Routers across the internet know how to forward packets to any public IP address.

Public IP addresses are scarce resources managed by Regional Internet Registries (RIRs) that allocate them to ISPs, organizations, and businesses according to need.

Private IP Addresses

Private IP addresses are reserved for use within local networks and cannot be routed on the public internet. Three ranges are designated for private use:

• 10.0.0.0/8 — 10.0.0.0 to 10.255.255.255
• 172.16.0.0/12 — 172.16.0.0 to 172.31.255.255
• 192.168.0.0/16 — 192.168.0.0 to 192.168.255.255

Your home router likely assigns addresses like 192.168.1.x to devices on your local network. These addresses are only meaningful within your home network—millions of other networks use the same private addresses simultaneously without conflict because they’re isolated from each other.

Network Address Translation (NAT)

NAT enables multiple devices with private IP addresses to share a single public IP address when accessing the internet. Your router translates between private addresses on your local network and the public address assigned by your ISP.

When your computer (192.168.1.100) requests a webpage, your router replaces your private address with its public address before sending the request to the internet. When the response returns, the router translates it back and forwards it to your computer.

NAT conserves public IPv4 addresses by allowing thousands of private addresses to operate behind each public address, extending IPv4’s usable lifespan despite address exhaustion.

Special IPv4 Address Ranges

Certain IPv4 address ranges serve special purposes beyond standard host addressing.

• Loopback addresses (127.0.0.0 to 127.255.255.255) allow a device to send network traffic to itself for testing purposes. The address 127.0.0.1 (commonly referred to as localhost) lets applications communicate with network services running on the same machine.

• Link-local addresses (169.254.0.0 to 169.254.255.255) are automatically assigned when a device can’t obtain an address from DHCP. If you see your computer using a 169.254.x.x address, it indicates DHCP failure or network connectivity issues.

• Broadcast addresses are the last address in each subnet, used to send data to all devices on that network simultaneously. In the network 192.168.1.0/24, the broadcast address is 192.168.1.255.

• Network addresses are the first address in each subnet, identifying the network itself rather than any specific host. In 192.168.1.0/24, the network address is 192.168.1.0.

Subnet Masks and CIDR Notation

The subnet masks determine which portion of an IP address identifies the network and which identifies the host.

Subnet Mask Format

Subnet masks use the same dotted decimal format as IP addresses. Common subnet masks include 255.255.255.0 (Class C default), 255.255.0.0 (Class B default), and 255.0.0.0 (Class A default).

In binary, subnet masks consist of consecutive 1s followed by consecutive 0s. The 1s indicate network bits, while 0s indicate host bits. The mask 255.255.255.0 in binary is 11111111.11111111.11111111.00000000, showing 24 network bits and 8 host bits.

CIDR Notation

CIDR (Classless Inter-Domain Routing) notation expresses subnet masks more concisely using a slash followed by the number of network bits.

The address 192.168.1.0 with subnet mask 255.255.255.0 is written as 192.168.1.0/24 in CIDR notation. The /24 indicates 24 bits are used for the network portion.

Common CIDR notations include /24 (255.255.255.0, 254 usable hosts), /16 (255.255.0.0, 65,534 usable hosts), and /8 (255.0.0.0, 16,777,214 usable hosts).

CIDR allows more flexible address allocation than the rigid class system, enabling networks of any size rather than only Class A, B, or C sizes.

Pro Tip: When calculating usable hosts in a subnet, remember to subtract 2 from the total addresses—one for the network address and one for the broadcast address. A /24 network has 256 total addresses but only 254 usable for hosts.

How IPv4 Addresses Are Assigned

Devices obtain IPv4 addresses through static assignment or dynamic assignment via DHCP.

Static IP Assignment

Static IP addresses are manually configured on each device and don’t change unless manually reconfigured. Servers, printers, network equipment, and other infrastructure devices typically use static addresses for consistent accessibility.

Administrators configure static addresses through the operating system’s network settings, specifying the IP address, subnet mask, default gateway, and DNS servers.

Static assignment ensures the device always uses the same address, which is important for services that other devices need to locate consistently.

Dynamic IP Assignment (DHCP)

DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses to devices when they connect to the network. Most home and office networks use DHCP for client devices like computers, phones, and tablets.

When a device connects, it broadcasts a DHCP request. The DHCP server (often your router) responds with an available IP address, subnet mask, default gateway, and DNS server information. The device uses this assigned address for a lease period, after which it can be renewed or a new address assigned.

DHCP simplifies network administration by eliminating manual configuration on each device and efficiently managing limited address pools by reusing addresses from disconnected devices.

For reliable web hosting with properly configured IPv4 addressing, subnetting, and network infrastructure, quality hosting providers ensure correct IP configuration and routing without requiring customers to manage low-level networking details.

IPv4 in Action: Routing and Communication

Understanding how IPv4 addresses enable communication clarifies their importance in networking.

When you visit a website, your computer needs the IP address of the destination server. DNS (Domain Name System) translates domain names into server IPv4 addresses. Your computer then creates data packets with your source IP address and the destination IP address in the packet headers.

Routers along the internet examine the destination IP address in each packet and forward it toward the destination network. Each router only needs to know the next hop closer to the destination, not the complete path.

This packet-switching approach using IP addresses allows data to travel across multiple interconnected networks reliably and efficiently, forming the foundation of internet communication.

IPv4 vs IPv6: What’s the Difference?

IPv4 uses 32-bit addresses, which provides about 4.3 billion possible address values. IPv6 uses 128-bit addresses, creating a vastly larger address space designed to support continued internet growth.

IPv6 does not simply replace IPv4 overnight. Many networks operate both protocols through dual-stack configurations or use transition technologies that allow IPv4 and IPv6 systems to communicate. For that reason, understanding IPv4 remains useful even as IPv6 adoption continues.

IPv4 Basics: Key Takeaways

IPv4 addresses are 32-bit numerical identifiers written in dotted decimal notation (like 192.168.1.1) that uniquely identify devices on networks. The 32-bit structure provides approximately 4.3 billion possible addresses, divided into network and host portions determined by subnet masks.

Address classes (A, B, C, D, E) originally organized IPv4 addresses by network size, though modern CIDR notation provides more flexible allocation. Public addresses are globally routable on the internet, while private addresses (10.x.x.x, 172.16.x.x, 192.168.x.x) work only within local networks, with NAT enabling private addresses to share public addresses.

IPv4 addresses are assigned statically through manual configuration or dynamically through DHCP servers. Subnet masks and CIDR notation define network boundaries and determine how many hosts each network supports. Understanding these IPv4 fundamentals provides the foundation for grasping networking concepts, troubleshooting connectivity issues, and appreciating why IPv6 was developed to overcome IPv4’s limitations.

Frequently Asked Questions