Names & addresses
What a DNS answer can tell you
Separate name resolution from reachability. Read the record type, think about caching, and avoid asking DNS to answer a question it cannot.
Start with
one question.
Short explanations of familiar things that are easy to take for granted. Each note makes one distinction and leaves you with something useful to check.
Requests & responses
Read the response before the page
Use the status line and a handful of headers to understand redirects, caches and the difference between a missing page and a missing connection.
Certificates & trust
What the little lock actually checks
Understand the name, the certificate chain and the clock. Then draw a clear line between a protected connection and a trustworthy website.