How IPv4 Subnetting Works: A Practical Guide With Worked Examples
Learn to find the network, broadcast, and host range of any IPv4 address by hand, size subnets for a given number of hosts, and split a network into equal or variable-sized pieces.
Networking and Security Guides
Practical guides for people who work with networks. Each one starts from a real problem, walks through the procedure step by step with commands and worked examples, explains how to interpret the results, and links to the NetEngToolkit tools that help.
How these guides are written: they are based on primary documentation (mainly the relevant RFCs, linked at the end of each guide), arithmetic examples are generated by the same code as the calculators, and sample output from live systems is labeled "illustrative". Each guide notes its last-updated date and states its limits. Start with whichever matches the problem in front of you.
Addressing and subnetting
CIDR Notation Explained: Prefixes, Masks, Ranges, and Summarization
Read and write CIDR notation confidently, convert between prefixes and masks, work out what a block covers, summarize routes, and turn address ranges into CIDR blocks.
DNS and web
How to Troubleshoot DNS Resolution Problems
A step-by-step method for diagnosing names that will not resolve, resolve to the wrong address, or work for some users and not others, using dig, nslookup, and public resolvers.
How to Diagnose HTTP Status Codes and Redirect Loops
Learn what the main HTTP status codes mean in practice, how to trace a redirect chain, and how to find the cause of redirect loops and 4xx/5xx errors.
How to Troubleshoot TLS Certificate Errors
Identify why a browser or client rejects a certificate (expired, wrong name, incomplete chain, untrusted issuer, protocol mismatch) and fix it, using openssl, curl, and certificate checks.
Email and security
How to Interpret SPF, DKIM, and DMARC Records
Understand what SPF, DKIM, and DMARC each prove, read the records and Authentication-Results headers, diagnose failures, and roll out DMARC safely.
How to Investigate a Suspicious IP Address or Domain
A structured first-pass triage workflow for an IP address, domain, URL, or email: gather observable facts, separate them from inference, correlate with your own logs, and decide what to do next.
Troubleshooting
How to Troubleshoot Network Connectivity Step by Step
A layered method for finding where a connection breaks: local configuration, gateway, routing, DNS, the path, the port, and the application, with commands for Windows, Linux, and macOS.