CIDR Calculator
Calculate a subnet's network address, broadcast address, mask, and usable host range — plus quick-reference tables for private, reserved, and special IP ranges.
Try it →Free tools for engineers
F12
A small toolbox, always growing
Small, honest tools for the everyday friction of infrastructure and ops work — checking an IP, working out a subnet, generating a password safely. No accounts, no upsells, no dark patterns.
Everything here runs client-side wherever that's possible, and says plainly what it does and doesn't send anywhere. If it saves one engineer five minutes at 2am, it's done its job.
Built and maintained by Denis Tolochko, an IT Systems Operations Engineer — say hello or browse the tools.
Why F12 exists
Small tools, built to actually get used.
Most engineering friction is small and repetitive: checking connectivity, working out a subnet, generating a password that's actually random. F12 is a growing set of tools for exactly that — fast, free, and built the way I'd want them built if I were the one reaching for them.
Connection & Browser Check
See exactly what your connection and browser expose to this site — IP, approximate location, network, TLS, and a live WebRTC/fingerprint leak test.
Try it →Domain Lookup
Look up a domain's DNS records and registration details in one place — A/AAAA/MX/NS/TXT/CAA records plus registrar, creation, and expiration dates.
Try it →Who's behind this
Denis Tolochko, IT Systems Operations Engineer.
Notes and research
Writing about infrastructure, operations, and the work behind the work.
Short technical notes, longer career reflections, and practical documentation from systems work.
Read the blog