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Work out the TCP MSS and maximum UDP payload for a given MTU, with the overhead of PPPoE, GRE, WireGuard, VXLAN and other tunnels subtracted.

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Convert an IPv4 address between dotted-decimal, 32-bit integer, hex and binary — and see whether it's private, public, loopback and more.

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Reformat a MAC address between colon, hyphen, Cisco dot and no-separator notation — and see its unicast/multicast and universal/local type.

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Resolve a relative reference against a base URI by RFC 3986, with the dot-segment trace and the browser’s answer beside it.

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Paste a DNS packet as hex or DoH base64url and read it: header flags, opcode and rcode, every record, and the EDNS0 OPT options.

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Paste a BIND zone file and see what it really says: every name fully qualified, the SOA explained, and the traps a parser alone will not report.

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Turn a DNSKEY into the DS record your registrar wants — key tag and all three digest types — or hash a name the way NSEC3 does.

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Turn a certificate into its TLSA records — every selector and matching type — and check an existing record against the certificate you are serving.

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Searchable cheat sheet for DNS record types — A, AAAA, CNAME, MX, TXT, NS, SOA, PTR, SRV, CAA, DNSSEC, SVCB / HTTPS — with examples.

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Searchable cheat sheet for ~60 standard TCP / UDP port numbers — from 22 (SSH) and 80 (HTTP) to 6379 (Redis) and 27017 (MongoDB).

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Type an IPv4 or IPv6 address and see its class, scope (private / public / loopback / link-local), decimal value, binary, reverse-DNS notation, and /32 CIDR.

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Build URLs by combining a base address with editable key-value query parameters — each pair toggleable, properly percent-encoded.

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Expand RFC 6570 URI templates with your own variables — all four levels, with path, query, fragment and label operators and explode modifiers.

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Convert any MAC address into its modified EUI-64 interface ID and the matching IPv6 link-local address — the same form Windows / Linux compute automatically.

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Toggle any IPv6 address between its shorthand ::1 form and the fully-expanded 8-group representation, with reverse DNS and binary too.

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Paste a raw email header to walk the Received hops in order, see the per-hop delay, and read off the SPF/DKIM/DMARC verdicts.

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Build an HTTP Cache-Control header from a visual checklist — freshness, revalidation, immutability, and the common presets.

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Build a CSP header from per-directive source lists, with three starter presets and live warnings for the dangerous wildcards.

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Compose Access-Control-* response headers from a checklist, with four presets and live warnings for the dangerous combinations.

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Paste a robots.txt and a list of user-agent and path pairs to see exactly which line allows or blocks each — with longest-match precedence.

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Compute the sha256 / sha384 / sha512 base64 hash an inline <script> or <style> block needs to be whitelisted by a Content-Security-Policy.

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Compose a Strict-Transport-Security header — max-age, includeSubDomains, preload — with a live preload-list eligibility check.

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Compose a Sender Policy Framework TXT record from mechanisms, IPs and includes — with a live DNS-lookup counter and warnings.

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Compose a _dmarc TXT record — policy, subdomain policy, percentage rollout, alignment, rua/ruf reporting — with safety warnings.

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Compose a Set-Cookie header from name and value plus the security attributes — SameSite, Secure, HttpOnly, Partitioned, Max-Age — with safety warnings.

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Strip utm_*, fbclid, gclid and 30+ other tracking params from a URL — paste, get a clean shareable link back.

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Encode username:password into the Base64 Authorization: Basic header — or paste an existing header back to see who's in it.

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Build the Permissions-Policy header: set camera, microphone, geolocation, payment, fullscreen and 20+ other browser features by origin allowlist.

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Pick a Referrer-Policy value and see exactly what Referer header outbound requests will send — same-origin, cross-origin HTTPS, HTTPS→HTTP downgrade.

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Split a long SPF, DKIM or DMARC TXT record into the 255-byte strings DNS requires, in BIND, generic zone-file, Cloudflare or Route 53 syntax.

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Expand a CIDR block (10.0.0.0/24) into a list of IPs, or convert a start-end range into the smallest covering CIDR set.

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Searchable catalog of well-known URIs registered with IANA — security.txt, openid-configuration, acme-challenge and 30+ more, with RFCs.

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Generate code_verifier + S256/plain code_challenge for OAuth 2.0 PKCE (RFC 7636).

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Estimate download time from file size and connection speed — or the speed required to transfer a given size within a deadline.

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Build or parse a DKIM DNS TXT record — paste your public key, pick selector, key type, hash and flags, and get the record plus its chunked form.

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Paste or load a sitemap.xml or sitemap index and see URL count, size, spec compliance, and the changefreq, priority and host distributions.

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Paste a feed and read it: entries, dates normalised, podcast enclosures, and the problems that make readers drop or duplicate posts.

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Paste an IANA cipher suite name and get its key exchange, authentication, bulk cipher, mode and MAC — plus a security grade for forward secrecy and AEAD.

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Paste a PEM certificate and read it: subject, issuer, expiry, SANs, key, fingerprints and extensions — decoded in your browser.

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Build a mailto: URL with To, Cc, Bcc, subject, and body — RFC 6068 percent-encoded, ready to drop into an <a> tag.

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Coalesce a messy list of IPv4/IPv6 addresses, CIDR blocks, and ranges into the smallest equivalent set of CIDR prefixes.

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Generate Certificate Authority Authorization (CAA) DNS records so only the CAs you trust can issue TLS certificates for your domain.

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Normalise, encode and check an SVCB or HTTPS record: SvcParams decoded, the wire bytes broken down, and every RFC 9460 rule it breaks named.

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