IPv4 Subnet Cheat Sheet

When you are planning or configuring a network you constantly need to look up the same basic facts — what subnet mask goes with /24? How many devices fit inside a /26? What wildcard mask do I enter for /28 in a Cisco ACL? Instead of calculating these by hand every time, this chart lists all 33 possible IPv4 prefix lengths from /0 to /32 with every key value pre-calculated in one place.

Find your prefix in the first column and read across the row to get the subnet mask, wildcard mask, total address count, usable host count, class, and a plain English description of where that prefix is typically used in real networks.

Here is a plain English explanation of every column so you know exactly what you are looking at:

1. →Prefix — The CIDR notation prefix length, written as a slash followed by a number from 0 to 32. This is the number after the slash in a network address like 192.168.1.0/24. /0 covers the entire IPv4 address space. /32 is a single specific device. Every value in between is a network of some size.
2. →Subnet Mask — The same information as the prefix but written in the four-number dotted decimal format — for example 255.255.255.0. Both formats describe exactly the same thing. Use this column when a device configuration screen has a mask field instead of a CIDR prefix field.
3. →Wildcard Mask — The exact bitwise opposite of the subnet mask. A 0 bit in the wildcard means this bit must match. A 1 bit means this bit can be anything. Use this column when writing Cisco access control lists and OSPF network statements. For /24 the wildcard is 0.0.0.255.
4. →Total Addresses — The complete size of the block including the reserved network and broadcast addresses. Calculated as 2 to the power of (32 minus the prefix). A /24 block has 2⁸ = 256 total addresses.
5. →Usable Hosts — The number of addresses you can actually assign to devices such as computers, phones, printers, or router interfaces. Always two less than Total Addresses — the first (network) and last (broadcast) are reserved. The exceptions are /31 with 2 usable and /32 with 1 usable — both explained in the section below.
6. →Class — The old classful network category. Class A is /8, Class B is /16, Class C is /24. Modern networks ignore these classes and use CIDR instead, but you will still see them in Cisco exams, textbooks, and older documentation — so the column is included for reference.
7. →Common Use — A brief description of where this prefix is typically used in real networks to help you quickly identify whether a given prefix is the right fit for what you are building.

The most common task when using this chart is finding the smallest subnet that comfortably fits the number of devices you need to connect. Here is how to do it in three steps with no maths at all:

1. 1Count the devices you need to connect on the segment — for example 45 computers and printers.
2. 2Scan the Usable Hosts column from the bottom of the chart upward. Stop at the first number that is equal to or greater than your device count. For 45 devices the first value large enough is 62 usable hosts — that is a /26.
3. 3Read across that row for the subnet mask 255.255.255.192, wildcard mask 0.0.0.63, and total block size 64 addresses. Everything needed to configure the segment is on one line.

This chart also helps you divide a larger block into smaller equal pieces. The maths is simple once you know the rule, and you can always verify the answer straight from the chart.

1. 1Subtract the parent (larger) prefix from the child (smaller) prefix to find how many bits you are borrowing. Example: fitting /26 subnets inside a /24 → 26 − 24 = 2 bits.
2. 2Raise 2 to the power of that number: 2² = 4 subnets. A /24 block contains exactly four /26 subnets.
3. 3Verify using the chart: each /26 has 64 total addresses. 4 × 64 = 256, which is exactly the Total Addresses value for /24. The numbers always multiply back to the parent block size when you have got it right.

Quick reference — how many subnets fit inside one /24 block

Three prefix lengths behave differently from every other row in the chart. They come up often in real network configurations and are worth understanding properly.

1. /0 The default route. Written as 0.0.0.0/0, a /0 covers all 4.3 billion IPv4 addresses at once. It is used as the catch-all last-resort route in routing tables. When a router cannot match a packet to any more specific route it forwards the packet to wherever the /0 entry points — almost always the internet gateway or firewall. Every router in the world has one.
2. /31 Point-to-point links between two routers. Defined in RFC 3021, a /31 has only two addresses. A point-to-point link connects exactly two devices, so there is no need for a network address or broadcast address — both addresses are fully usable. This saves two IP addresses per link compared to using a /30 for the same purpose, which matters when you have thousands of router interconnects across a large network.
3. /32 A single specific host. A /32 is the address space of exactly one device. It is used for loopback interfaces on routers — a Loopback0 interface with a /32 stays up as long as the router is running and is used as a stable identifier for BGP sessions and management access. It is also used in firewall rules when a rule should apply to exactly one IP address and nowhere else.

A CIDR prefix is the number after the slash in a network address such as 192.168.1.0/24. It tells you how many of the 32 bits in an IPv4 address are used to identify the network, with the remaining bits identifying individual devices within that network. A /24 uses 24 bits for the network and leaves 8 bits for hosts — giving 256 total addresses and 254 usable ones. A larger prefix number means a smaller network with fewer hosts. A smaller prefix number means a larger network with more hosts.

Every subnet has two addresses that cannot be assigned to any device. The first address is the network address — it identifies the subnet itself and appears in routing tables. The last address is the broadcast address — sending a packet to it delivers it to every device on the subnet at once. Usable hosts always equals total addresses minus 2. A /24 has 256 total but only 254 usable. A /28 has 16 total but only 14 usable. The only exceptions are /31 and /32 — both are explained in the question below.

A subnet mask uses 1 bits to mark the network portion and 0 bits to mark the host portion. A wildcard mask is the exact bitwise opposite — it has 0 bits where the subnet mask has 1 bits, and 1 bits where the subnet mask has 0 bits. In a wildcard mask a 0 means this bit must match and a 1 means this bit can be anything. For /24 the subnet mask is 255.255.255.0 and the wildcard is 0.0.0.255. Wildcard masks are used in Cisco ACL lines and OSPF network statements. The chart shows both so you always have the right format for the right context.

A /32 has only one address in the block — it represents a single specific device and is used for router loopback interfaces and individual host routes in BGP. There is no separate network or broadcast address. A /31 has exactly two addresses and is defined in RFC 3021 specifically for point-to-point links between two routers. Because both addresses are needed for the two connected devices there is no room for a network or broadcast address, so both addresses are treated as fully usable. This is why the chart shows Usable Hosts as 2 for /31 and 1 for /32 rather than following the standard minus-2 rule.

A /0 prefix means zero bits are used for the network portion, so the address block covers all 4,294,967,296 IPv4 addresses in the entire internet at once. Written as 0.0.0.0/0, it is used as the default route in routing tables. When a router receives a packet and cannot find any more specific matching route, it falls back to the /0 default route and forwards the packet there — usually toward the internet gateway or firewall. You will see 0.0.0.0/0 in nearly every router configuration pointing to the upstream provider.

Subtract the parent prefix from the child prefix to find how many bits you are borrowing, then raise 2 to that power. To find how many /26 subnets fit inside a /24: 26 − 24 = 2 bits borrowed, 2² = 4 subnets. Verify using the chart: each /26 has 64 total addresses, and 4 × 64 = 256 which exactly matches the /24 Total Addresses value. This multiplication check always works — the child block sizes multiplied by the number of subnets will always equal the parent block size if the maths is right.

The prefixes you encounter most often are /24 for a standard office or home LAN — by far the most common in the world; /30 for point-to-point router links where only 2 usable addresses are needed; /32 for loopback interfaces and host routes; and /0 as the default route. When slicing a /24 into smaller pieces for different departments or VLANs, the /25, /26, /27, /28, and /29 prefixes are regularly used. The /16 and /8 prefixes appear in large campus, enterprise, and ISP networks where tens of thousands of addresses are needed under one block.

Before CIDR was introduced, IPv4 addresses were grouped into fixed classes based on the first octet. Class A used a /8 mask giving up to 16,777,214 usable hosts — assigned to very large organisations and governments. Class B used a /16 mask giving 65,534 usable hosts — assigned to universities and large companies. Class C used a /24 mask giving 254 usable hosts — the most common for smaller businesses. Today classful networking has been entirely replaced by CIDR, which allows any prefix length and makes address allocation far more flexible and efficient. The Class column in the chart is included purely as a reference because you will still see these terms in Cisco CCNA exams, textbooks, and older equipment documentation.