[ Free tool · Generators ]
SPF CIDR Calculator
Work out an IP range and the exact ip4:/ip6: SPF mechanism.
[ About this tool ]
This SPF CIDR calculator works out the network, netmask, first and last address and address count for any IPv4 or IPv6 range, and gives you the exact ip4: or ip6: mechanism.
How to read the results
- ›Use the network address, not a host inside the range.
- ›Wide ranges such as /8 let others send as you.
- ›Only authorise IP ranges you control.
Frequently asked questions
What does /24 mean in SPF?
ip4:203.0.113.0/24 authorises all 256 addresses from 203.0.113.0 to 203.0.113.255.
Learn more in our guide: SPF (Sender Policy Framework)
Enterprise provenance
Why top financial institutions and governments retired their email security dashboards.
Traditional email authentication platforms sell you a dashboard and ask your engineers to do the labour. In high-stakes environments, manual oversight means delayed enforcement, configuration drift and unmonitored shadow domains.
EmailAuth is the agentic email provenance platform. Our agents enforce p=reject, keep third-party senders aligned and move your domains to full RFC 9989 compliance, the new DMARC standard,, with human-in-the-loop auditability.
SaaS · Dedicated tenancy · Air-gapped on-prem · BYOK encryption
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