Networking
OSI/TCP-IP, IP and ports, DNS, HTTP, and how to diagnose it when something's broken.
Basics
Seven OSI layers, four TCP/IP layers. Why these numbers show up in load balancer names.
What an IP address and subnet mask are, public and private addresses, NAT (SNAT/DNAT), what ports are for, and why 80 and 443 are special.
How google.com turns into an IP address. A/AAAA/CNAME/MX/TXT records, dig, /etc/resolv.conf.
Request/response, methods, status codes, headers. HTTP/1.1 vs 2 in two paragraphs.
ping, traceroute, mtr, ss, curl -v, tcpdump. Which one to use for which symptom.
iptables/nftables, ufw, the routing table, and why ping/traceroute stay silent even though the service is alive.
Three-way handshake, sequence numbers, SYN/ACK/FIN/RST flags, and what makes TCP reliable.
How UDP works, what you trade for speed, and why DNS, video, and games pick it.
What ICMP carries, how ping and traceroute work, and why blocking ICMP entirely causes trouble.
How address translation works: getting a private network onto the internet, forwarding ports, and what NAT isn't.
How IPv6 differs from IPv4 beyond address length, what link-local means, and how a host configures its own address.
The DORA sequence, what else a DHCP server hands out besides an IP address, and the lifecycle of a lease.
Eleven questions on OSI, DNS, HTTP, TCP, NAT, DHCP, and diagnostics.
Going deeper
How HTTPS sets up encryption: the TLS handshake, certificates, the chain of trust, and what the browser checks.
Where free certificates come from, how the ACME domain challenge works, and why auto-renewal is mandatory.
Why you put a proxy in front of an application: TLS termination, routing by domain, headers, and what X-Forwarded-For is for.
Why spread traffic across multiple servers: algorithms, health checks, and the L4 vs L7 difference.