Enter any IPv4 address into this free network class identifier to instantly determine its IP class (A, B, C, D, or E), whether it is a private or public address, its default subnet mask, its routing scope, and any special properties such as loopback, link-local (APIPA), broadcast, or multicast. This tool is essential for networking students, IT professionals, and anyone studying for CompTIA Network+, CCNA, or similar certifications.
Understanding IP address classes is fundamental to networking. Although modern networks use CIDR (Classless Inter-Domain Routing) rather than classful addressing, the original class system is still referenced everywhere — in textbooks, certification exams, network documentation, and routing configurations. This tool gives you instant, accurate classification for any IPv4 address you encounter.
Enter an IPv4 address to identify its class, scope, and special properties.
IP address classes are a method of dividing the IPv4 address space into ranges with different intended uses and different default subnet masks. The classful addressing system was established in the early days of the internet when it was assumed that the entire address space would be divided into large blocks assigned to organizations. Each class defined how many bits were used for the network portion versus the host portion of an IP address.
The system was designed with a hierarchy in mind: Class A addresses were allocated to very large organizations (like major ISPs and governments) that needed millions of host addresses. Class B addresses served medium-to-large organizations like universities and corporations. Class C addresses were for small networks with up to 254 hosts. Classes D and E were reserved for special purposes.
The class of an IP address is determined entirely by the value of the first octet (the first 8 bits). This made class identification trivially fast for routers in an era when processing power was limited. A router only needed to look at the first octet to know how to interpret the rest of the address and what the default subnet mask should be. For today's common private addresses like 192.168.1.1 (Class C) and 10.0.0.1 (Class A), the class system still tells you the default mask and helps you understand how the address was originally intended to be used.
The classful system was formally replaced by CIDR in 1993 (RFC 1519), but understanding it remains essential for any networking professional because it forms the conceptual foundation for everything that came after. If you need to work with subnet calculations, our Subnet Calculator tool handles both classful and CIDR notations.
The IPv4 address space (0.0.0.0 to 255.255.255.255) is divided into five classes. Each class is defined by the leading bits of the first octet and has a specific default subnet mask and intended use:
| Class | First Octet Range | Default Mask | Max Hosts per Network | Typical Use |
|---|---|---|---|---|
| A | 1 – 126 | /8 (255.0.0.0) | 16,777,214 | ISPs, large organizations, governments |
| B | 128 – 191 | /16 (255.255.0.0) | 65,534 | Universities, mid-size companies |
| C | 192 – 223 | /24 (255.255.255.0) | 254 | Small networks, home routers |
| D | 224 – 239 | N/A | N/A | Multicast groups |
| E | 240 – 255 | N/A | N/A | Reserved for research / future use |
Note that 127.x.x.x is technically within the Class A range (1–126 is Class A proper), but it is reserved entirely for loopback use. The address 127.0.0.1 (called "localhost") always refers to the local machine and never routes over a network. This is why routers and networking equipment treat 127.x.x.x as a special case separate from the Class A definition.
The host count formula for classful networks is 2^(host bits) – 2, where the two subtracted addresses are the network address (all host bits set to 0) and the broadcast address (all host bits set to 1). For Class A, that's 2^24 – 2 = 16,777,214. For Class C, it's 2^8 – 2 = 254. Understanding this is foundational to subnetting — use our Subnet Calculator to work through CIDR subnet divisions based on these foundations.
Not all IP addresses can be routed on the public internet. RFC 1918 defines three ranges of "private" IP addresses that are reserved exclusively for use within private networks (homes, offices, data centers). These addresses are not routable on the internet — any router on the internet will drop packets addressed to RFC 1918 addresses without forwarding them.
| Range | CIDR Notation | Class | Total Addresses | Common Use |
|---|---|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | A | 16,777,216 | Large enterprises, VPNs, data centers |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | B | 1,048,576 | Medium enterprises, Docker networks |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | C | 65,536 | Home routers, small offices |
The reason private addresses exist is NAT (Network Address Translation). Your home router uses NAT to share a single public IP address among all your devices by mapping private IP addresses to the public IP. When you check what your IP address is, you see the public IP assigned by your ISP — not the private 192.168.x.x address on your home network. To understand the full picture of how this works, read our guide on what is NAT and how it enables private addressing to work at internet scale. You can find your router's private IP (its gateway address) using our guide on finding your router IP address.
Beyond the five main classes and the RFC 1918 private ranges, there are several special-purpose IP address ranges that networking professionals need to recognize instantly:
| Address / Range | Name | Purpose | Routable? |
|---|---|---|---|
| 127.0.0.0/8 | Loopback | Always refers to local machine; used for testing local software | No |
| 169.254.0.0/16 | Link-Local (APIPA) | Auto-assigned when DHCP fails; only valid on local segment | No |
| 255.255.255.255 | Limited Broadcast | Sends to all hosts on the local network; used by DHCP discovery | No |
| 224.0.0.0 – 239.255.255.255 | Multicast | Delivers packets to a group of subscribed receivers simultaneously | Varies |
| 0.0.0.0 | Unspecified / Default Route | Represents "any" or "unknown" address; used in routing tables | No |
| 100.64.0.0/10 | Shared Address Space | Carrier-grade NAT (CGN); used by ISPs between CPE and BRAS | No (internal to ISP) |
The 169.254.x.x range (APIPA — Automatic Private IP Addressing) is particularly important to recognize. If a device on your network shows an IP in this range, it means DHCP failed — the device could not reach a DHCP server to get a proper IP address. Common causes include a disconnected network cable, a router that's offline, or a DHCP service that has crashed. This is often the first sign to look for when diagnosing network connectivity problems. Read our guide on what is DHCP for a complete explanation of how automatic IP assignment works.
The classful addressing system worked well in the early internet when the number of connected organizations was small. However, it had a fundamental inefficiency problem: the granularity was too coarse. An organization that needed 300 host addresses had to be assigned a full Class B network (65,534 addresses), wasting over 65,000 addresses. An organization needing 500 addresses got a Class B even though they only needed two Class C networks worth of space.
By the early 1990s, the internet was growing so fast that IPv4 addresses were being consumed rapidly, and routing tables were exploding in size as each Class C network required a separate entry. Two solutions emerged simultaneously: CIDR (Classless Inter-Domain Routing, RFC 1519 in 1993) and NAT (RFC 1631 in 1994).
CIDR replaced the fixed class boundaries with variable-length subnet masking (VLSM). Instead of assigning a /8, /16, or /24 based on class, any organization could be assigned exactly the size block they needed — a /22 for 1,022 hosts, a /27 for 30 hosts, a /20 for 4,094 hosts. This dramatically slowed IPv4 address exhaustion and dramatically reduced routing table size through route aggregation (supernetting). Modern networks use CIDR notation exclusively, but understanding the class foundations makes CIDR easier to learn and work with. For CIDR calculations, use our Subnet Calculator.
The quickest way to tell if an IP address is private (RFC 1918) is to check if it falls within one of the three reserved ranges: 10.x.x.x, 172.16.x.x through 172.31.x.x, or 192.168.x.x. Any address outside these three ranges (and outside the other special ranges like loopback and link-local) is a public, internet-routable address.
In practice, devices on your home network typically have addresses in the 192.168.x.x range (the most common home router default). Your router itself has both a private IP (its LAN address, which you use to log in at the router admin page) and a public IP (assigned by your ISP, which is what the internet sees). When you use our What Is My IP tool, you see the public address your ISP assigned. When you check your device's network settings, you see the private address your router's DHCP server assigned. Understanding what a gateway is explains how these two different IP worlds connect together.
For network administrators managing larger deployments, it helps to remember that the 10.x.x.x range is the most flexible private range because it provides the largest single contiguous block (16 million addresses), making it ideal for large enterprise networks and VPN infrastructure. The 172.16-31.x.x range is commonly seen in cloud and container environments (Docker uses 172.17.0.0/16 by default). The 192.168.x.x range is nearly universal in homes and small offices.
Pro Tip: When troubleshooting network connectivity issues, the first thing to check is what IP address a device has received. If it shows 169.254.x.x (APIPA), DHCP failed — check cables, router status, and the DHCP server lease pool. If it shows 10.x.x.x or 192.168.x.x, it has a valid private IP and the issue is elsewhere. If it shows a public IP directly on the device (no NAT), you're in a special network segment. Use the Class Identifier tool above to classify any suspicious IP addresses you encounter during troubleshooting.
Key Takeaways
IP address classes are a legacy system for dividing the IPv4 address space into five ranges (A through E), each with different default subnet masks and intended uses. Class is determined by the first octet: 1–126 = A (large networks), 128–191 = B (medium networks), 192–223 = C (small networks), 224–239 = D (multicast), 240–255 = E (reserved). The classful system was replaced by CIDR in 1993 but remains fundamental networking knowledge.
192.168.1.1 is a Class C IP address. The first octet (192) falls in the 192–223 range that defines Class C. It is also a private IP address under RFC 1918 (192.168.0.0/16 range), meaning it is not routable on the public internet. The default subnet mask for a Class C network is 255.255.255.0 (/24). This is the most common default router IP address used in home networking — see our 192.168.1.1 guide for router login details.
10.0.0.1 is a Class A IP address. The first octet (10) falls in the 1–126 range that defines Class A. It is also a private IP address under RFC 1918 (the 10.0.0.0/8 range), giving it over 16 million possible addresses. The default subnet mask for a Class A network is 255.0.0.0 (/8). The 10.x.x.x range is widely used in enterprise networks, VPNs, and data centers. See our 10.0.0.1 router guide for routers that use this as a default gateway.
Class D addresses (first octet 224–239) are reserved for IP multicast. Unlike unicast addresses (Classes A–C) that identify a single host, a multicast address identifies a group of receivers that have opted in to receive traffic for that group. Multicast is used for efficient delivery of streaming media, routing protocol updates (OSPF uses 224.0.0.5, RIP uses 224.0.0.9), and IPTV. Class D addresses do not have subnet masks because they represent group memberships, not individual hosts.
Classful addressing wasted enormous numbers of IP addresses because allocations had to be in fixed /8, /16, or /24 blocks. An organization needing 500 hosts had to receive a Class B network with 65,534 addresses, wasting 65,034 addresses. CIDR (Classless Inter-Domain Routing) allows any prefix length (e.g. /22 for 1,022 hosts), allocating only what's needed. CIDR also enables route aggregation, which dramatically reduces internet routing table size and makes routing more efficient.
A private IP address is one from the three RFC 1918 ranges that are not routable on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Devices on your home or office network use private IPs assigned by your router's DHCP server. Your router uses NAT to translate between these private addresses and the single public IP address assigned by your ISP. Private addresses can be reused across millions of different private networks worldwide without conflict because they never appear on the public internet.
169.254.x.x is the APIPA (Automatic Private IP Addressing) range, also called link-local. When a device's DHCP request fails — because the DHCP server (usually your router) is unreachable — the device automatically assigns itself an address in the 169.254.0.0/16 range using a process called APIPA. These addresses are only valid on the local network segment and cannot communicate with other networks or the internet. Seeing a 169.254.x.x address on a device almost always means there's a DHCP connectivity problem to investigate.
127.0.0.1 is the loopback address, also known as "localhost." Any traffic sent to this address is routed back to the same device — it never leaves the machine. The entire 127.0.0.0/8 range is reserved for loopback, though 127.0.0.1 is by far the most commonly used. It's widely used by developers to test local web servers and applications, and by networking software to test that TCP/IP is functioning on a device without needing network hardware.
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About Tommy N.
Tommy is the founder of RouterHax and a network engineer with 10+ years of experience in home and enterprise networking. He specializes in router configuration, WiFi optimization, and network security. When not writing guides, he's testing the latest mesh WiFi systems and helping readers troubleshoot their home networks.
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