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Guides
Practical, no-fluff explanations of the network protocols and records you actually have to debug. Each guide pairs with a tool on this site so you can run the lookup yourself.
The Codynet guides are a growing library covering the network plumbing you actually have to debug — DNS resolution and TTLs, email authentication with SPF, DKIM, and DMARC, TLS certificates and chain-of-trust errors, RDAP registration records, and IPv4 and IPv6 subnetting. Each one is written for the moment something is broken and you need to understand why it behaves the way it does, not for passing an exam.
Every guide pairs with a live tool on this site, so you can read how a record or protocol is supposed to work and then run the exact lookup against your own domain in the same session. The goal is to turn a confusing symptom — a bouncing email, an untrusted certificate, a record that resolves differently depending on who is asking — into a specific, checkable answer instead of guesswork.
DNS
How records resolve, what TTLs mean, and how to read resolver errors.
DNS Records Explained · NXDOMAIN vs SERVFAIL: What DNS Errors Actually Mean · DNS TTL Explained: What Is TTL in DNS? · What Is A CAA Record? · nslookup vs dig: Which DNS Tool Should You Use? · Reading A Full dig +trace Response · DNS Not Resolving? A Step-By-Step Checklist
Make sure mail is routed and authenticated — MX, SPF, DKIM, and DMARC.
MX Records And Email Routing · SPF, DKIM, And DMARC Explained · SPF Record Failing? How To Read It In DNS
TLS
Keep certificates valid, trusted, and correctly chained.
TLS Certificate Expiration And Troubleshooting · TLS Certificate Chain Explained · Why Your New SSL Certificate Shows A Name-Mismatch Error
RDAP
Look up who registered a domain or which network owns an IP.
Network
Address space, CIDR notation, and subnet math.
All guides
DNS
DNS Records Explained
A practical guide to A, AAAA, CNAME, MX, TXT, NS, CAA, and other DNS record types.
7 min read · Updated 2026-07-08MX Records And Email Routing
How mail exchanger (MX) records control email routing for a domain, and how to troubleshoot MX routing when mail delivery breaks.
6 min read · Updated 2026-07-08SPF, DKIM, And DMARC Explained
A domain owner's guide to the DNS records that help receivers trust email.
8 min read · Updated 2026-06-02SPF Record Failing? How To Read It In DNS
How to find your SPF record in DNS, read every mechanism and qualifier, and fix the real failures: softfail to spam, the 10-lookup permerror, and duplicate records.
7 min read · Updated 2026-07-08TLS
TLS Certificate Expiration And Troubleshooting
How to inspect certificate dates, names, issuers, and trust problems before users see errors.
7 min read · Updated 2026-05-06DNS
NXDOMAIN vs SERVFAIL: What DNS Errors Actually Mean
What is NXDOMAIN and what is SERVFAIL? NXDOMAIN means the DNS name does not exist; SERVFAIL means the resolver could not get a clean answer. Two different DNS errors, two different fixes.
5 min read · Updated 2026-06-18RDAP
RDAP vs WHOIS: What Replaced WHOIS And Why
WHOIS is a 40-year-old plaintext protocol. RDAP returns the same data as structured JSON over HTTPS. Here's what each one is and when to use which.
6 min read · Updated 2026-06-18DNS
DNS TTL Explained: What Is TTL in DNS?
DNS TTL (time to live) tells resolvers how long to cache a record. Learn what TTL means in DNS, sensible default TTL values, and how to set TTL before a migration.
5 min read · Updated 2026-06-18DNS
What Is A CAA Record?
A CAA record tells the world's CAs which of them are allowed to issue certificates for your domain. Cheap insurance against mis-issuance.
5 min read · Updated 2026-05-14DNS
nslookup vs dig: Which DNS Tool Should You Use?
nslookup vs dig — what is the difference? nslookup is the older, simpler DNS lookup tool; dig shows the full DNS response for troubleshooting. Here's when to use each.
6 min read · Updated 2026-06-18DNS
Reading A Full dig +trace Response
How to read a dig +trace from the root servers down to your authoritative nameservers, and how it exposes delegation and lame-nameserver problems a normal lookup hides.
7 min read · Updated 2026-07-08DNS
DNS Not Resolving? A Step-By-Step Checklist
An ordered checklist to isolate whether a DNS failure is your machine, your resolver, or the domain — using the error code and a public resolver test.
8 min read · Updated 2026-07-08Network
Subnetting For Beginners: CIDR, Masks, And /24 Math
Subnetting is just counting in binary. This guide walks through what /24 means, how CIDR works, and why /30 is the smallest useful IPv4 subnet.
8 min read · Updated 2026-05-14HTTP
HTTP Security Headers Explained
What HTTP security headers do, which ones every site should send, and a sensible baseline for HSTS, CSP, nosniff, frame protection, Referrer-Policy, and Permissions-Policy.
8 min read · Updated 2026-06-29TLS
TLS Certificate Chain Explained
How leaf, intermediate, and root certificates build a chain of trust — and why a missing intermediate breaks some clients but not others.
7 min read · Updated 2026-07-08TLS
Why Your New SSL Certificate Shows A Name-Mismatch Error
A name-mismatch error means the hostname is not in the certificate's Subject Alternative Names. Covers apex vs www, wildcard depth, wrong cert, and half-deployed renewals.
7 min read · Updated 2026-07-08