IP Address and Its Classes: A Complete Guide

Every device on a network needs an address, the same way every house on a street needs one so mail can find it. On the internet, that address is the IP address. If you have ever opened a router’s admin page and seen a string like 192.168.1.1, or typed ipconfig into a command prompt and stared at a number you didn’t fully understand, this guide is for you. We’ll cover what an IP address is, how it is written, and how the older “classful” system split addresses into Classes A through E.

What Is an IP Address?

An IP address (Internet Protocol address) is a unique number assigned to every device connected to a network, whether that’s your phone on Wi-Fi, a server in a data center, or a printer in an office. It is what lets devices find each other and exchange data.

There are two versions in use today:

• IPv4: a 32-bit address, usually written as four numbers separated by dots, like 192.168.1.10. Each of the four numbers (called an octet) can range from 0 to 255.
• IPv6: a newer, 128-bit address written in hexadecimal and split into eight groups separated by colons. IPv6 exists because IPv4 simply ran out of room, but IPv4 is still what most people mean when they say “IP address,” and it’s the focus of this article.

Dotted Decimal Notation

A 32-bit binary number is not something anyone wants to read or type, so IPv4 addresses are written in dotted decimal notation instead. The 32 bits are split into four 8-bit sections (octets), and each octet is converted to decimal.

A few rules to keep in mind:

• Each of the four segments must fall between 0 and 255.
• Leading zeros aren’t allowed. 054 is invalid; 54 is fine.
• The same address can also be written in hexadecimal (base 16), where each byte becomes a two-digit hex value, though this is rarely used outside of specific technical contexts.

Why IP Addresses Were Split Into Classes

In the early days of the internet, from 1981 to 1993, addresses were assigned using a system called classful addressing. The idea was simple: divide the 32-bit address space into a handful of fixed-size classes, so that a network could get either a small block, a medium block, or a huge block of addresses depending on its size, without needing complicated subnetting.

This worked well when the internet was small. A router could look at the very first few bits of an address and instantly know which class it belonged to and how to route it, which made routing tables fast and simple to manage.

The Five Classes of IP Addresses

Classful addressing splits the 32-bit space into five classes: A, B, C, D, and E. Classes A, B, and C are used for regular device-to-device (unicast) communication and cover the vast majority of addresses you’ll ever run into. Class D is reserved for multicasting, and Class E was set aside for research and experimentation.

Class A

Class A is built for networks that need an enormous number of hosts. Only the first bit of the address is fixed (it’s always 0), which leaves 7 bits for the network ID and a full 24 bits for host IDs.

• First octet range: 0 to 127 (with 0 and 127 reserved for special use, so usable addresses run from 1 to 126)
• Default subnet mask: 255.0.0.0
• Usable hosts per network: 2²⁴ − 2 = 16,777,214

Because a single Class A network can hold over 16 million hosts, these blocks were historically handed out to large ISPs and major organizations rather than individual companies.

Class B

Class B addresses always start with the bits 10, splitting the address into a 16-bit network ID and a 16-bit host ID.

• First octet range: 128 to 191
• Default subnet mask: 255.255.0.0
• Number of networks: 2¹⁴ = 16,384
• Usable hosts per network: 2¹⁶ − 2 = 65,534

This type of IP B sits in the middle: too big for a home network, but a reasonable fit for a mid-sized company or a large private network.

Class C

IP addresses in Class C start with the bits 110. The network ID takes up 24 bits, leaving only 8 bits, one octet, for host addresses.

• First octet range: 192 to 223
• Default subnet mask: 255.255.255.0
• Number of networks: 2²¹ = 2,097,152
• Usable hosts per network: 2⁸ − 2 = 254

This is the class you’ll actually recognize. Home routers, small offices, and most local networks use Class C-style addressing (think 192.168.1.x), simply because 254 hosts is plenty for a household or a small business.

Class D

Class D breaks from the network ID/host ID pattern entirely. It starts with the bits 1110, and the remaining 28 bits identify a multicast group rather than a specific host.

• First octet range: 224 to 239
• Purpose: multicasting, sending one packet to many interested recipients at once, rather than to one device (unicast) or to every device on the network (broadcast)
• No subnet mask is defined for Class D.

Class E

Class E starts with the bits 1111 and was reserved for experimental and research use. Like Class D, it has no network/host split and no subnet mask.

• First octet range: 240 to 255

Quick Reference Table

Class	Leading Bits	First Octet Range	Default Subnet Mask	Usable Hosts / Network
A	0	1 – 126	255.0.0.0	16,777,214
B	10	128 – 191	255.255.0.0	65,534
C	110	192 – 223	255.255.255.0	254
D	1110	224 – 239	Not defined	Reserved for multicast
E	1111	240 – 255	Not defined	Reserved for research

Rules for Assigning Network and Host IDs

A few restrictions apply no matter which class an address falls into:

• A host ID of all zeros represents the network itself, not an actual device, so it can’t be assigned to a host.
• A host ID of all ones is reserved for the broadcast address, used to reach every host on that network at once.
• A network ID can’t start with 127. That entire range is set aside for loopback testing (127.0.0.1 being the most familiar example, your own machine talking to itself).
• A network ID of all zeros refers to the current, local network and isn’t routed anywhere.

Special IP Address Ranges Worth Knowing

• 127.0.0.0 – 127.255.255.255: loopback addresses, used to test a device’s own network stack.
• 169.254.0.0 – 169.254.255.255: link-local addresses, assigned automatically when a device can’t reach a DHCP server.
• 0.0.0.0/8: represents “this network,” typically seen before a device has been assigned a real address.

A Word on Subnet Masks

A subnet mask tells a device which part of an IP address is the network portion and which part identifies the host. Take the mask 255.255.255.0. In binary that’s 11111111.11111111.11111111.00000000. Lining it up against an address like 192.168.123.132 splits the address cleanly: the first three octets (192.168.123) form the network address, and the last octet (132) identifies the specific host on that network.

Why Classful Addressing Didn’t Last

Classful addressing was simple, but simplicity came at a cost. A company that needed 300 addresses had no good option: a Class C network only offered 254 hosts, so it wasn’t enough, but the next size up, Class B, handed over 65,534 addresses, wasting more than 65,000 of them. Multiply that across thousands of organizations, and you get an internet running out of address space far faster than it should have. That inefficiency is what pushed the industry toward Classless Inter-Domain Routing (CIDR) in 1993, which allows networks to be sized far more precisely instead of being locked into one of five fixed boxes. Classful addressing isn’t used for real-world routing anymore, but understanding it still matters. It’s the foundation every networking course builds on, and concepts like subnet masks and address ranges only make sense once you’ve seen where they came from.

Published September 2026. Last verified: September 2026.