What does /0 mean in CIDR?

0.0.0.0/0 represents all IPv4 addresses (the entire internet). It's commonly used as the default route in routing tables and as a catch-all in firewall rules. Never use /0 in allow rules for sensitive resources.

How do I check if an IP is in a CIDR block?

Apply the subnet mask to both the CIDR base address and the target IP using bitwise AND. If the results match, the IP is in the block. For example, 10.0.5.100 AND 255.255.240.0 = 10.0.0.0, matching 10.0.0.0/20.

Can CIDR blocks overlap?

10.0.0.0/16 fully contains 10.0.1.0/24. Overlapping CIDRs cause routing ambiguity. Most cloud providers reject overlapping subnets within a VPC. When planning, map all subnets to verify non-overlap.

What is the minimum CIDR block for AWS VPCs?

AWS VPCs support /16 (65,536 IPs) to /28 (16 IPs). Subnets within VPCs also follow this range. Other clouds have similar constraints. Always check provider documentation for current limits.

How do I aggregate multiple CIDR blocks?

CIDR aggregation (supernetting) combines adjacent blocks into a larger block. For example, 10.0.0.0/24 and 10.0.1.0/24 can aggregate to 10.0.0.0/23. This reduces routing table entries and simplifies firewall rules.

Variable Length Subnet Masking (VLSM) allows different subnets to use different prefix lengths within the same network. Instead of uniform /24 subnets, you might use /28 for a small subnet and /20 for a large one, using address space more efficiently.

CIDR to IP Range Calculator

Convert CIDR notation to first and last IP addresses, total host count, and subnet mask. Supports all IPv4 prefixes.

IP AddressEnter network base address

Prefix Length

/20

Network Address

10.0.0.0

First address in subnet

Broadcast Address

10.0.15.255

Last address in subnet

First Usable Host

10.0.0.1

Gateway typical

Last Usable Host

10.0.15.254

Last assignable host

Total Addresses

4,096

All IPs in range

Usable Hosts

4,094

Available for devices

Network Notation

CIDR Notation

10.0.0.0/20

Standard subnet notation

Host Bits

2^12 = 4,096 addresses

Wildcard Mask

4095

Inverse of subnet mask

Subnet Details

Type	IP Address	Decimal Value
Network	10.0.0.0	0
First Host	10.0.0.1	1
Last Host	10.0.15.254	4,094
Broadcast	10.0.15.255	4,095

Common Prefix Sizes

16,777,214 hosts

/16

65,534 hosts

/24

254 hosts

/25

126 hosts

/26

62 hosts

/27

30 hosts

/28

14 hosts

/29

6 hosts

/30

2 hosts

Planning notes, formulas, and examples

About the CIDR to IP Range Calculator

CIDR (Classless Inter-Domain Routing) notation compactly represents an IP address range as a base address and prefix length: for example, 192.168.1.0/24 represents the 256 addresses from 192.168.1.0 to 192.168.1.255. This calculator expands any CIDR block to show the full IP range.

Converting CIDR to IP ranges is essential for configuring firewalls, security groups, access control lists, and network monitoring tools that accept IP ranges. It is also useful for verifying that a specific IP is within a given CIDR block.

This calculator supports all IPv4 prefixes from /0 (the entire IPv4 space) to /32 (a single host). It computes the network address (first IP), broadcast address (last IP), and all usable addresses in between.

When This Page Helps

Firewall rules, security groups, and ACLs often require explicit IP ranges. This calculator converts CIDR notation to the corresponding first and last IP addresses.

How to Use the Inputs

Enter a base IP address (e.g., 192.168.1.0).
Enter the CIDR prefix length (e.g., 24).
The calculator shows the first IP, last IP, and total addresses.
Use the range for firewall rules and access control lists.

Formula used

Network Address = IP AND subnet_mask
Broadcast = Network Address OR (~subnet_mask)
First Usable = Network + 1
Last Usable = Broadcast − 1
Total = 2^(32 − prefix)

Example Calculation

Result: Range: 10.0.0.0 – 10.0.15.255 (4,096 addresses)

A /20 has 12 host bits: 2^12 = 4,096 addresses. The network address 10.0.0.0 with mask 255.255.240.0 spans to 10.0.15.255. This is a common VPC subnet size in AWS for large auto-scaling groups.

Tips & Best Practices

Use CIDR notation in security groups — it's more concise than IP ranges.
The base IP should be the network address (aligned to the prefix boundary).
Common cloud prefixes: /16 for VPCs, /24 for subnets, /32 for single hosts.
Verify CIDR blocks don't overlap before creating network peering connections.
/0 matches all IPs (0.0.0.0/0) and is used as the default route.
Always double-check CIDR ranges before applying to firewall rules.

Understanding CIDR Notation

CIDR replaced the old classful addressing system (Class A, B, C) with flexible prefix lengths. This enables more efficient IP address allocation and more granular routing. Modern networking relies entirely on CIDR for address management.

CIDR in Cloud Networking

Cloud VPCs are defined by CIDR blocks. When creating a VPC, you specify a primary CIDR (e.g., 10.0.0.0/16). Subnets within the VPC use smaller CIDRs (e.g., 10.0.1.0/24). Security groups and NACLs use CIDR for source/destination filtering.

Common CIDR Ranges

RFC 1918 defines three private ranges: 10.0.0.0/8 (16M addresses), 172.16.0.0/12 (1M addresses), and 192.168.0.0/16 (65K addresses). Use these for internal networks. Cloud providers also offer 100.64.0.0/10 (Carrier-Grade NAT) for some internal services.

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

0.0.0.0/0 represents all IPv4 addresses (the entire internet). It's commonly used as the default route in routing tables and as a catch-all in firewall rules. Never use /0 in allow rules for sensitive resources.