Browser, device and network tools

Check records, devices and the browser you are using. DNS, email-authentication and speed checks contact the public service named on each page; a single browser test measures one path at one moment and does not diagnose a whole network.

Inspect records and connection information

DNS record lookup
Look up A, AAAA, MX, TXT, CNAME, NS, SOA, CAA and SRV records.
Uses a public service
SPF, DKIM and DMARC checker
Check a domain's SPF, DKIM and DMARC records and explain problems.
Uses a public service
IP subnet calculator
IPv4 and IPv6 subnets: network, broadcast, host range, mask and splitting.
Internet speed test
Measure download, upload and latency to Cloudflare's speed-test network.
Uses a public service

Check browser and input-device behavior

Passwords

Password and passphrase generator
Generate strong random passwords or word-based passphrases, with entropy shown.

Guides

Understanding DNS records: A, AAAA, CNAME, MX, TXT and NS: What each common DNS record type does, how to read a zone with real examples, how TTL and caching work, and the mistakes that break websites and email.
SPF, DKIM and DMARC in plain English: What the three email authentication records do, how to read them, and the most common mistakes that cause mail to be rejected.
IPv4 subnetting step by step: CIDR, masks and usable hosts: How to work out network, broadcast and host ranges by hand, with worked /26, /27 and /20 examples and a method for splitting a network into subnets.
Strong passwords and passphrases: what entropy means: How to judge password strength in bits of entropy, why length beats complexity, and when a passphrase is the better choice.