Log in
CaptainDNS

Propagation & diagnostics

Compare resolvers worldwide and inspect returned answers.

Email authentication

Tools to verify and validate your email authentication setup.

SPF syntax validator

Review include chains and DNS query limits before publishing changes.

SPF record inspector

Resolve the published TXT record, flag lookup limits and decode each mechanism.

DKIM syntax validator

Quickly check the syntax of your DKIM records.

DKIM record check

Resolve a selector and analyse the published DKIM TXT record.

DMARC syntax validator

Check rua/ruf destinations, alignment modes and the active policy before publishing.

DMARC record inspector

Resolve the published policy, surface diagnostics and confirm enforcement before switching to reject.

BIMI syntax validator

Validate BIMI tags, the logo fetch and the VMC certificate before publishing.

BIMI record inspector

Resolve the published BIMI record, inspect logo and VMC diagnostics in one place.

Email deliverability audit

Audit MX, SPF, DKIM and DMARC to confirm a domain is ready to send.

Mail header analyzer

Decode participants, authentication results and routing from raw headers.

DMARC record generator

Create compliant DMARC records with all policy and reporting options.

Text

Transform, encode and measure your content in seconds.

Text case converter

Convert any block of text to upper or lower case instantly.

Base64 encoder / decoder

Encode or decode any content in Base64 without leaving the browser.

Slug generator

Transform any sentence into an SEO-friendly slug in seconds.

Word & character counter

Measure the length of any text, with instant word and character counts.

Images

Preview and validate your BIMI logos before publishing.

BIMI logo lookup

Inspect a BIMI logo URL—format, metadata and live rendering—before rollout.

Certificates

Inspect CSRs, BIMI logos and VMC certificates before you deploy.

CSR parser

Inspect a CSR, extract subject details, fingerprints and requested SANs.

VMC inspector

Inspect a Verified Mark Certificate—issuer, validity and SAN coverage—before you publish BIMI.

IP

Look up addresses, owners, reverse records and ranges.

Reverse lookup

Resolve a PTR record and validate DNS consistency.

IP WhoIs

Identify the owner of an IP range and its contacts.

IPv4 netmask

Calculate network, broadcast and usable hosts for any IPv4 block.

My IP address

Detect your IPv4/IPv6 addresses and their geolocation.

IPv4 netmask calculator

Why use this calculator?

Calculate in one click the network, mask, broadcast and usable address range of an IPv4.
Useful for setting an ACL, choosing a gateway, creating a VLAN, documenting an addressing plan or checking a configuration provided by an operator.
The tool accepts a single IP with CIDR prefix 192.168.1.42/24 or a network/prefix pair 192.168.1.0/24.

What the tool returns

  • Mask in CIDR and decimal notation 255.255.255.0
  • Network address and broadcast address
  • First and last usable addresses
  • Number of usable hosts
  • "Wildcard mask" 0.0.0.255 useful for some ACLs

How to use

  1. Enter an address in IPv4 format or directly a CIDR block.
  2. Click Calculate.
  3. Read the summary and copy what you need.

Tip
If you enter an IP with a prefix 203.0.113.9/30, the calculation is done on the block to which this IP belongs.

Quick reminders about CIDR

  • /24 equals 255.255.255.0 and contains 256 addresses of which 254 are usable.
  • /30 block of 4 addresses. 2 usable. Ideal for router to router links.
  • /31 block of 2 addresses. 2 usable without broadcast on RFC 3021 compatible equipment.
  • /32 single address. Used to designate a host.

Concrete examples

192.168.1.42/24
Network 192.168.1.0
Broadcast 192.168.1.255
Usable range 192.168.1.1 → 192.168.1.254
Hosts 254

203.0.113.9/30
Network 203.0.113.8
Broadcast 203.0.113.11
Usable range 203.0.113.9 → 203.0.113.10
Hosts 2

Private ranges and special addresses

These addresses are not routed on the Internet. They are designed for internal networks.

Private

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

169.254.0.0/16

Loopback

127.0.0.0/8

CGNAT operator side

100.64.0.0/10

Documentation and public testing

192.0.2.0/24
198.51.100.0/24
203.0.113.0/24

Multicast

224.0.0.0/4

Historical reserved

0.0.0.0/8
240.0.0.0/4

Limited broadcast

255.255.255.255

Good to know
A private address behind a box cannot directly receive connections from the Internet without translation and appropriate rules.

When to use it?

  • Size a subnet and know exactly the usable range.
  • Check that a block provided by a service provider matches your host needs.
  • Prepare firewall rules with network, mask and wildcard.
  • Check an addressing plan before deployment or migration.
  • Explain an outage due to a broadcast IP or an incorrect mask.

Best practices

  • Choose a prefix that leaves a reasonable margin of hosts. Neither too wide nor too tight.
  • Avoid using the network address and broadcast address.
  • Clearly reserve the gateway. In practice often .1 or .254, but it's up to you.
  • Note the TTL of DNS records if you change the addressing of an exposed service.
  • Document each network block and the allocation date.

Points of attention

  • /31 is not universal. Check support on your routers before use.
  • Inconsistent masks on either side of a link create random losses.
  • IP geolocation databases do not depend on the mask. A change of operator can move the perceived location.
  • Private and CGNAT ranges are not valid for publishing a public service.