Network & DNS
Technical guides about network & dns, written for practical follow-up.
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13 articlesWhy your public IP changes: VPNs, CGNAT and IPv6
Trace an unexpected address to the request, route and address family that produced it, with a practical comparison record.
Read DNS records when a website or email service stops working
Use A, AAAA, CNAME, MX and TXT evidence to isolate delegation, record and cache problems before changing a zone.
IPv4 subnetting with a worked /26 network plan
Calculate network boundaries, usable ranges and broadcast addresses, including the exceptions for point-to-point links and host routes.
Read IPv6 addresses, prefixes and compressed notation
Expand and shorten IPv6 correctly, recognise address scope and separate a local address observation from internet reachability.
DNSSEC validation: read the AD flag and the chain behind it
Distinguish resolver assertions, signed zones and broken delegation evidence before changing DS or DNSKEY records.
Domain names, Unicode and Punycode: inspect the exact hostname
Separate a domain label, URL and registrable name, then review internationalised names without treating conversion as a trust verdict.
CAA records: authorise certificate issuers without blocking renewals
Review issuer tags, wildcard handling and inherited policies before a certificate renewal reaches its deadline.
Plan a DNS migration around TTLs and real cache behaviour
Build a cutover timeline that accounts for old TTLs, negative caching, resolver policy and application connections.
Audit an email domain using a real delivered message
Follow envelope sender, DKIM signing domain and visible From through SPF, DKIM and DMARC evidence.
SPF records: lookup limits, includes and the wrong From address
Trace SPF evaluation, identify expensive DNS mechanisms and understand why SPF success can still leave DMARC failing.
DMARC alignment and a staged path to enforcement
Use authentication reports and representative mail flows to decide when quarantine or reject is appropriate.
DKIM selectors, public keys and rotation without guesswork
Locate the selector in a real signature, inspect its DNS key and distinguish key publication from message verification.
A VPN check needs more than an IP address.
HTTP, WebRTC and DNS can follow different paths. Learn to compare the evidence without mistaking one successful check for a complete audit.
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13 guides