Subnet math, DNS lookups, DHCP options, and certificate checks share one browser tab in Networking Toolbox, a free toolkit for sysadmins and network engineers. Need to carve a /24 into departments? The VLSM planner fits named subnet requirements into a parent block, reports each subnet's efficiency and the leftover space, and exports the plan as CSV or JSON. Merging address lists? CIDR utilities summarize, split, compare, and deaggregate prefixes, with set operations for difference, overlap, and containment. Writing configs? Generators build DHCPv4 and DHCPv6 options, PXE profiles, Kea or ISC snippets, and SPF, DKIM, DMARC, CAA, TLSA, and SRV records, and DNSSEC helpers calculate key tags and DS records. Something broken? Diagnostics query DNS through Cloudflare, Google, Quad9, OpenDNS, or a resolver of your choice, read TLS certificate chains, grade HTTP security headers, trace redirects, check TCP ports, look up BGP routes and RDAP records, and test mail servers. Need a refresher? Reference guides explain private ranges, CGNAT, IPv6 address types, and common ports. A keyboard search palette puts any tool a few keystrokes away, and bookmarked tools stay available offline. Eighteen themes, four languages, custom branding, and accessibility switches make it feel like your own, and a keyless JSON API serves the subnet, CIDR, IP, and DNS utilities to scripts. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. MIT licensed.