CIDR calculator: Free online tool for email security

CIDR calculator overview:
  • Convert CIDR to readable IP ranges in seconds so you can confidently add the right ip4: mechanisms to your SPF record.
  • Avoid overly broad SPF authorizations by seeing which IP addresses a CIDR includes (and tightening ranges where needed).
  • Support better email security and deliverability with accurate DNS entries that reduce accidental exposure and make spoofing harder.
The Sendmarc CIDR calculator converts CIDR notation into readable IP ranges that you can plug straight into your DNS to support accurate email delivery. It also acts as a simple subnet calculator for day-to-day network planning. Sendmarc Cidr Calculator Tool Preview
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What is CIDR notation?

Classless Inter-Domain Routing (CIDR) notation is a way of writing IP address ranges in a compact format that combines an IP address and a prefix length. For example, 127.0.0.1/15 consists of:
  • An IP address: 127.0.0.1
  • A slash and prefix length: /15
In CIDR, an IP address has two parts: The network address, which shows the network or subnet, and the host identifier, which points to a specific device on that network. CIDR notation is used throughout routing, firewall rules, cloud networking, and email authentication. A CIDR calculator removes the need to manually convert between this notation and address ranges.

Why a CIDR calculator is useful

A CIDR calculator is a focused type of subnet calculator. It helps you understand what’s inside an IP range, such as 127.0.0.1/15, without doing any manual work.

For email security, a CIDR calculator is especially helpful when you configure your Sender Policy Framework (SPF) record. SPF uses mechanisms such as ip4: and ip6: to describe which IP addresses can send email for your domain. If you add ip4:127.0.0.1/15 to your SPF record, you authorize every address in that range. A CIDR calculator shows you which IPs are included and helps you decide whether the range is wider than necessary.

Using a CIDR calculator as part of your SPF and Domain-based Message Authentication, Reporting, and Conformance (DMARC) configuration helps you keep IP ranges tight and DNS records accurate. That reduces the risk of accidental exposure and makes spoofing more difficult.

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How the Sendmarc CIDR calculator works

The Sendmarc CIDR calculator is designed for technical users who want clear results without extra complexity. It fits naturally into email infrastructure and security workflows. You can use the calculator in two main ways:

1. CIDR to IP range

You enter an IP address and a prefix length. For example, you enter 127.0.0.1/15. The calculator derives the network properties. The Sendmarc CIDR calculator then shows information such as:
  • CIDR range
  • CIDR base
  • Host
  • Broadcast
  • Block size
  • First IP
  • Last IP

2. IP range to CIDR

You can also start with a range of IP addresses. The calculator works out the CIDRs that cover that range as efficiently as possible. This is useful when you know which systems should be included, and you want to express them in compact CIDR notation.

Analyze SPF records after using the CIDR calculator

Defining IP ranges correctly is an important step, but you still need to see how those decisions behave inside an SPF record. After you use the CIDR calculator to refine your IP ranges, Sendmarc can help you analyze the SPF record itself.

You can paste your current SPF record into the Sendmarc analyzer to:

  1. Check for common issues, such as too many DNS lookups
  2. Spot overlapping or redundant mechanisms

In practice, the workflow is simple. Generate CIDRs, update SPF, and then validate the record with the analyzer.

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Get started with the Sendmarc CIDR calculator

Accurate CIDR planning is a small detail that has a large impact on email delivery and security. When IP ranges are clearly defined and correctly reflected in an SPF record, it becomes harder for attackers to impersonate your domain.

Sendmarc combines a practical CIDR calculator, intelligent SPF and DMARC analysis, and ongoing monitoring to give you a complete view of how your domains are used. Sendmarc’s enterprise-grade platform is specifically built for complex, multi-provider environments.

If you want to move from one-off configuration changes to continuous protection, Sendmarc can support that journey.

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CIDR calculator FAQs

What is CIDR notation?

CIDR notation is a compact way of writing IP address ranges by combining an IP address with a prefix length, such as 127.0.0.1/15. The number after the slash shows the network or subnet and the host identifier. CIDR notation lets you represent large or small IP ranges without listing every address.

The best CIDR notation for SPF records is the one that closely matches the IP addresses that are actually allowed to send email on behalf of your domain. In practice, this means using CIDR blocks that include all of your legitimate mail servers, but avoid very broad ranges that include systems you don’t control.

The Sendmarc CIDR calculator is currently designed for IPv4 address ranges. If you need to plan IPv6 ranges, you can still apply the same CIDR concepts and use dedicated IPv6 planning tools.