Free, in your browser, no sign-up. These are the same engines that run inside the Kodura apps, so an answer here is the answer you get on your phone.
One report on a domain: the website certificate, whether email from it can be faked, and the DNS basics. Where something needs fixing, it gives you the record to paste, with your mail provider filled in.
Paste a CIDR block, a subnet mask, or an entire ifconfig
dump: it reads them all and works out the rest. No retyping octets into
four little boxes.
Reads the certificate chain exactly as your server sends it, with no silent repairs, so a missing intermediate shows up here even when the site looks fine in your browser.
Queries Cloudflare and Google side by side and tells you whether they agree, which is how you catch a record that has propagated to one resolver and not the other.
Checks whether a TCP port is reachable from the public internet, and tells a refused connection apart from a filtered one, which is the half of the answer most checkers throw away.
Fetches a URL and says whose problem the answer is. A status code proves the network worked: the failures that really are the network do not have one.
Counts SPF's ten-lookup limit and shows which include spent
it. A record can be perfectly valid and still fail every message once it
goes over: this says whether yours has.
Asks two independent resolvers at once and shows the TTL on each answer, the number that actually says how much longer a stale answer can survive.
Finds the manufacturer in the IEEE registry and reads the address itself. Many MAC addresses have no vendor at all, and that is a fact about the address rather than a gap in the data.
001b.6384.45e6Racks, rack density and a PUE become IT load, facility load and annual energy. It separates power from energy and installed capacity from actual draw, which is where published figures usually go wrong.
An IT load becomes heat. This estimates the water needed to move that heat for all four cooling designs at once, with the energy each one costs alongside it.
A browser can do more networking than people expect, but not everything. Subnetting is pure arithmetic, so it runs on your own machine with nothing sent anywhere. DNS-over-HTTPS is a public API, so the lookup queries the resolvers directly from your browser. A TLS handshake needs a raw socket, so the SSL checker runs on our server.
A browser cannot send ICMP, which rules out ping, traceroute and path MTU discovery. Any web page offering those is measuring the path from its server, not from you, which is rarely the thing you wanted to know. Those live in Kodura Network Tools for iPhone and iPad, where they measure your actual connection.
Stuck on something specific? The writing works through the errors these tools are usually reached for.