Headers, queries, subnets and the wire. All 6 tools below run locally in your browser — nothing is uploaded.
This is the debugging half of web work: what a URL actually contains, what your browser is really sending, which addresses a CIDR block covers, and what a server meant by the status code it returned.
The reference tools matter as much as the calculators. HTTP status codes and header semantics are things most people look up rather than memorise, and having them searchable beside the parsers means one fewer tab open on a spec you only need two lines from.
256 total, 254 usable. The IP & CIDR tool gives the network and broadcast addresses, the usable range and the mask for any prefix you enter.
The Headers & UA tool shows your user agent and the client information available to a page, which is the quickest way to confirm what a server is seeing from your side.
I'm a teapot — an April Fools' code from 1998 that stuck around. The status code reference is searchable and covers every standard code with what it actually signals.