Subnet Cheat Sheet: CIDR, Subnet Masks & IP Ranges

March 31, 2026 · 12 min read

Understanding IP subnetting is fundamental for network engineers, system administrators, and anyone working with network infrastructure. This comprehensive subnet cheat sheet provides quick reference tables for CIDR notation, subnet masks, IP address ranges, and usable host calculations—everything you need to design, troubleshoot, and optimize your networks.

Whether you're configuring a home router, designing an enterprise network, or studying for your CCNA certification, bookmark this page for instant access to subnet calculations and best practices.

Understanding Subnetting Basics

Subnetting is the practice of dividing a network into smaller, more manageable sub-networks. This technique improves network performance, enhances security, and makes IP address allocation more efficient.

Every IPv4 address consists of 32 bits, divided into a network portion and a host portion. The subnet mask determines where this division occurs. For example, in the address 192.168.1.100 with mask 255.255.255.0, the first three octets identify the network, while the last octet identifies the host.

CIDR (Classless Inter-Domain Routing) notation provides a compact way to represent subnet masks. Instead of writing 255.255.255.0, you can write /24, indicating that 24 bits are used for the network portion.

Complete IPv4 Subnet Reference Table

This comprehensive table shows all standard subnet sizes from /32 (single host) to /8 (Class A network). Use this as your go-to reference for quick lookups during network design and troubleshooting.

Calculating Usable Hosts Per Subnet

The formula for calculating the number of usable hosts in a subnet is straightforward but essential to understand:

Usable Hosts = 2^(32 - prefix) - 2

The -2 accounts for two reserved addresses in every subnet: the network address (all host bits set to 0) and the broadcast address (all host bits set to 1). These addresses cannot be assigned to individual hosts.

Example calculation for /26:

• Host bits: 32 - 26 = 6 bits
• Total addresses: 2^6 = 64
• Usable hosts: 64 - 2 = 62

There are two important exceptions to this rule:

1. /31 subnets: RFC 3021 allows point-to-point links to use both addresses, giving 2 usable hosts instead of 0
2. /32 subnets: Represents a single host route, commonly used in routing tables and loopback addresses

IPv4 Address Classes (Legacy)

Before CIDR was introduced in 1993, IPv4 addresses were divided into classes based on the first octet. While classful networking is obsolete, understanding these classes helps when working with legacy systems and documentation.

Why classful networking became obsolete: The rigid class structure led to massive IP address waste. A company needing 300 hosts would receive a Class B network with 65,534 addresses, wasting 65,234 addresses. CIDR solved this by allowing flexible subnet sizes.

Note that 127.0.0.0/8 is reserved for loopback addresses and is not part of Class A. The address 127.0.0.1 is the standard loopback address used to refer to the local machine.

Private IP Address Ranges

RFC 1918 defines three IP address ranges reserved for private networks. These addresses are not routable on the public internet, making them ideal for internal networks. Internet routers are configured to drop packets with private IP addresses as source or destination.

Choosing the right private range:

• Use 10.0.0.0/8 for large organizations with multiple sites or complex network hierarchies
• Use 172.16.0.0/12 for medium-sized networks or when 10.0.0.0/8 conflicts with VPN or partner networks
• Use 192.168.0.0/16 for home networks, small offices, or isolated lab environments

When connecting to VPNs or merging networks, ensure your private IP ranges don't overlap with the remote network's addressing scheme. This is a common source of connectivity issues.

Special-Use IP Addresses

Beyond private addresses, several IP ranges are reserved for special purposes. Understanding these ranges prevents configuration errors and helps with troubleshooting.

Loopback addresses (127.0.0.0/8): Used to refer to the local machine. The most common is 127.0.0.1, but any address in this range loops back to the local host.

Link-local addresses (169.254.0.0/16): Automatically assigned when DHCP fails. If you see these addresses, it indicates a DHCP configuration problem. Also known as APIPA (Automatic Private IP Addressing) in Windows environments.

Multicast addresses (224.0.0.0/4): Used for one-to-many communication. Common examples include 224.0.0.1 (all hosts on subnet) and 224.0.0.2 (all routers on subnet).

Broadcast address: The last address in any subnet, used to send packets to all hosts on that subnet. For 192.168.1.0/24, the broadcast address is 192.168.1.255.

Documentation addresses (RFC 5737): Three ranges reserved for documentation and examples:

• 192.0.2.0/24 (TEST-NET-1)
• 198.51.100.0/24 (TEST-NET-2)
• 203.0.113.0/24 (TEST-NET-3)

Carrier-grade NAT (100.64.0.0/10): Reserved for shared address space in service provider networks (RFC 6598).

Practical Subnetting Examples

Let's work through real-world subnetting scenarios to demonstrate how to apply these concepts in practice.

Example 1: Small Office Network

You need to set up a network for an office with 40 computers, 5 printers, and 10 IP phones (55 devices total).

• Required hosts: 55
• Recommended subnet: /26 (62 usable hosts)
• Network: 192.168.1.0/26
• Usable range: 192.168.1.1 to 192.168.1.62
• Broadcast: 192.168.1.63

This provides room for 7 additional devices without reconfiguration.

Example 2: Multi-Department Building

You're designing a network for a building with four departments: Sales (30 hosts), Engineering (50 hosts), HR (15 hosts), and IT (20 hosts).

Using VLSM (covered in the next section), you can efficiently allocate subnets:

• Engineering: 10.1.1.0/26 (62 hosts)
• Sales: 10.1.1.64/27 (30 hosts)
• IT: 10.1.1.96/27 (30 hosts)
• HR: 10.1.1.128/27 (30 hosts)

Example 3: Point-to-Point Router Links

You need to connect three routers with point-to-point links. Use /30 subnets to minimize address waste:

• Link 1: 10.0.0.0/30 (Router A: 10.0.0.1, Router B: 10.0.0.2)
• Link 2: 10.0.0.4/30 (Router B: 10.0.0.5, Router C: 10.0.0.6)
• Link 3: 10.0.0.8/30 (Router A: 10.0.0.9, Router C: 10.0.0.10)

Each link uses only 4 addresses, with 2 usable for the router interfaces.

Variable Length Subnet Masking (VLSM)

VLSM allows you to use different subnet masks within the same network, enabling more efficient IP address allocation. This technique is essential for modern network design.

Benefits of VLSM:

• Reduces IP address waste by matching subnet size to actual requirements
• Allows hierarchical network design
• Supports route summarization for more efficient routing tables
• Provides flexibility for network growth and changes

VLSM Design Process:

1. List all network segments and their host requirements
2. Sort segments from largest to smallest
3. Assign subnets starting with the largest, ensuring no overlap
4. Document your addressing scheme thoroughly

VLSM Example: Starting with 172.16.0.0/16, allocate subnets for:

• Data center: 500 hosts → 172.16.0.0/23 (510 hosts)
• Office LAN: 200 hosts → 172.16.2.0/24 (254 hosts)
• Guest WiFi: 100 hosts → 172.16.3.0/25 (126 hosts)
• Management: 50 hosts → 172.16.3.128/26 (62 hosts)
• DMZ: 10 hosts → 172.16.3.192/28 (14 hosts)
• Router links: 2 hosts each → 172.16.3.208/30, 172.16.3.212/30, etc.

This approach uses address space efficiently while maintaining room for growth in each segment.

Supernetting and Route Aggregation

Supernetting (also called route aggregation or summarization) combines multiple smaller networks into a larger one. This technique reduces routing table size and improves network performance.

Example: If you have four consecutive /24 networks, you can summarize them into a single /22:

• 192.168.0.0/24
• 192.168.1.0/24
• 192.168.2.0/24
• 192.168.3.0/24

These can be summarized as 192.168.0.0/22, which represents all four networks in a single routing table entry.

Requirements for successful supernetting:

• Networks must be contiguous (consecutive)
• The number of networks must be a power of 2 (2, 4, 8, 16, etc.)
• The first network must start on an appropriate boundary

Route aggregation is crucial for ISPs and large enterprises to keep routing tables manageable. Without it, internet backbone routers would need to maintain millions of individual routes.

Common Subnetting Mistakes to Avoid

Even experienced network engineers occasionally make subnetting errors. Here are the most common pitfalls and how to avoid them.

1. Forgetting to subtract 2 from host calculations

Always remember that the network and broadcast addresses cannot be assigned to hosts. A /24 has 256 total addresses but only 254 usable hosts.

2. Overlapping subnets

When using VLSM, ensure your subnets don't overlap. Use a spreadsheet or network documentation tool to track your allocations. Our Subnet Calculator can help verify that subnets don't conflict.

3. Not planning for growth

Always allocate more addresses than you currently need. A good rule of thumb is to provision for 30-50% growth over the next 3-5 years.

4. Using the wrong subnet mask

Double-check your subnet mask calculations. A single bit error can cause connectivity issues that are difficult to troubleshoot. For example, 255.255.255.0 is /24, not /25.

5. Ignoring private IP ranges

Using public IP addresses for internal networks can cause routing conflicts and security issues. Always use RFC 1918 private addresses for internal networks.

6. Misconfiguring point-to-point links

For router-to-router connections, use /30 or /31 subnets. Using larger subnets like /24 wastes hundreds of addresses per link.

7. Not documenting the addressing scheme

Maintain clear documentation of your subnet allocations, including purpose, VLAN assignments, and contact information. Future you (and your colleagues) will be grateful.

8. Forgetting about special addresses

Remember that 0.0.0.0, 127.0.0.0/8, 169.254.0.0/16, and multicast ranges have special meanings and shouldn't be used for regular host addressing.

Frequently Asked Questions