DNS Lookup

Choose a DNS record type to search for.

Propagation & diagnostics

Compare resolvers worldwide and inspect returned answers.

Propagation testPublic resolvers, remaining TTLs, divergent responses.
Domain DNS auditParent/local NS, authority and response times.
DNSSEC CheckerValidate chain of trust, DS records and signing keys.
API & integrations
Keep history, monitor your zones and automate recurring checks.

Email authentication

Tools to verify and validate your email authentication setup.

Syntax validation

Record inspectors

Generators

Deliverability

Email deliverability audit Mail header analyzer Email tester IP Blacklist Checker Domain Blacklist Checker SPF Flattener DKIM Selector Finder SMTP/MX Tester

Hosting

MTA-STS hosting BIMI Hosting

IP tools

Quickly inspect an IP address or your own connection.

Text

Transform and measure your content in seconds.

Text case converter

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

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.

Password generator

Generate strong random passwords and memorable passphrases instantly.

Developer

Encoding, hashing, regex and formatting for everyday dev work.

Base64 encoder / decoder

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

Hash Generator

Compute MD5, SHA-1, SHA-256 and SHA-512 hashes of any text.

URL encoder / decoder

Encode or decode text using percent-encoding (RFC 3986) right in your browser.

Regex Tester

Test a regular expression against text and visualize matches.

JSON / YAML Formatter

Format, validate and convert JSON and YAML in one click.

Web & Certificates

BIMI logos, VMC/CSR certificates and web page diagnostics.

BIMI logo lookup

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

BIMI SVG converter

Convert any SVG to BIMI-compliant Tiny-PS format in seconds.

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.

Page size checker

Check if your page exceeds Google's 2 MB crawl limit.

Phishing URL Checker

Check if a URL is flagged as phishing or malware by 4 threat intelligence sources.

IPv6

IPv6 adoption, dual-stack configuration, and migration strategies.

2 articles

February 2026

Visual comparison of IPv6 prefixes /48, /56, and /64 with subnet counts

CaptainDNS · February 19, 2026

IPv6 subnets: understanding /48, /56, and /64

Which IPv6 prefix should you use for your network? Detailed comparison of /48, /56, and /64 sizes with a table, decision tree, and real-world examples.

December 2025

Diagram comparing an IPv4 address and an IPv6 address on a network background

CaptainDNS · December 10, 2025

IPv4 vs IPv6: understand the differences and plan the transition

IPv4 vs IPv6: address sizes, performance, security, NAT, DNS... Learn how to differentiate ipv4 and ipv6, what ipv6 vs ipv4 really means, the ipv4 and ipv6 difference for DNS, and how to keep both stacks coexisting.