Core Calculators

CIDR Calculator

IPv4, IPv6, overlap detection, supernet aggregation, conversions — everything you need in one place.

IPv4 Calculator
Prefix /16 65,536 total addresses
Network
—
Broadcast
—
Usable hosts
—
Total IPs
—
Subnet mask
—
Wildcard mask
—
First usable
—
Last usable
—
IP class
—
Range type
—
In-addr.arpa
—
Binary, Hex & Decimal Conversions
Binary
—
Hexadecimal
—
Decimal (integer)
—
IPv4-mapped IPv6
—
IPv6 Calculator
Network prefix
—
Prefix length
—
Expanded
—
Compressed
—
Total addresses
—
/64 subnets
—
Address type
—
Scope
—
Overlap Detection
Supernet / CIDR Aggregator
IP Range to CIDR

When to reach for the CIDR calculator

The calculator answers the most common subnetting questions instantly: given a CIDR block, what's the network address, what's the broadcast, how many hosts can you put inside, and what does the binary look like? For a fresh subnet design from scratch or a single network to inspect, this is the right starting point.

If you're slicing one parent block into many child subnets of different sizes, you'll want the VLSM planner instead — it handles host-count-driven allocation with no math. For visualizing how subnets fit inside their parent, the subnet visualizer renders the same data as bitmaps, heatmaps, or VPC topology diagrams. To check whether your input is RFC-compliant or might collide with reserved space, run it through the validator.

The overlap check, specifically

A surprisingly common bug in production networks is two CIDRs that quietly overlap. Routing tables don't error out — they just deliver packets to whichever route was installed last or has the longer prefix. The overlap check on this page tells you immediately whether two blocks are identical, one contains the other, partially overlap, or are fully disjoint. If you're doing this kind of validation at scale across many subnets, the IPAM tool tracks an inventory and flags new conflicts as you add allocations.

Background reading

If subnet math itself is still a bit fuzzy, the how-to-calculate-subnets walkthrough covers the binary mechanics step by step. The subnet mask cheat sheet is a quick prefix-to-mask-to-host-count reference. For ACL and firewall work, subnet mask versus wildcard mask explains why Cisco-style wildcards are an inverted netmask and when each is appropriate.

Related tools

VLSM Planner
Allocate child subnets by host count from a parent block.
Visualize
Tree, bitmap, heatmap, and VPC topology views.
Validate
RFC compliance and reserved-range checks.
Cloud-Aware
AWS, Azure, GCP, and OCI reserved-IP handling.

Frequently asked questions

How many usable hosts are in a /24 subnet?

A standard /24 subnet has 254 usable hosts. The /24 contains 256 total addresses, but the first address is the network address and the last is the broadcast address, both reserved. In cloud environments the usable count is lower: AWS and Azure give you 251 usable hosts in a /24, GCP gives 252.

What is the difference between a subnet mask and a wildcard mask?

A subnet mask uses 1 bits for the network portion and 0 bits for the host portion (e.g., 255.255.255.0 for a /24). A wildcard mask is the bitwise inverse (e.g., 0.0.0.255 for a /24). Cisco access control lists use wildcard masks; most other contexts use subnet masks. Both describe the same network boundary.

How do I check if two CIDR blocks overlap?

Two CIDR blocks overlap if their address ranges share at least one IP. The overlap tool on this page returns one of five results: identical, A contains B, B contains A, partial overlap, or separate. Enter both blocks in CIDR notation and the tool computes the relationship instantly.

Can I use /31 for a point-to-point link?

Yes. RFC 3021 permits /31 subnets for point-to-point links, treating both addresses as usable. A /31 has only 2 addresses total, and on point-to-point links there is no broadcast traffic, so the traditional network-address and broadcast-address reservations are not needed. /31 is the modern choice for router-to-router and other strictly point-to-point connections.

What is supernet aggregation?

Supernet aggregation combines multiple contiguous CIDR blocks into a single, larger block with a shorter prefix. For example, 192.168.0.0/24 and 192.168.1.0/24 aggregate into 192.168.0.0/23. Aggregation reduces routing table size and is the foundation of CIDR's main benefit over classful addressing.

Is 100.64.0.0/10 a private range like 10.0.0.0/8?

No. 100.64.0.0/10 is reserved for Carrier-Grade NAT under RFC 6598. It is allocated for use inside ISP networks for CGNAT deployments, not for enterprise or home networks. Using CGNAT space in your own network can cause routing conflicts with your ISP. For private networks, use the RFC 1918 ranges: 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16.