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Portal | Level: L1: Foundations | Topics: TCP/IP, DNS, VLANs, Routing | Domain: Networking

Networking Fundamentals - Skill Check

Mental model (bottom-up)

Networking is layers: bits -> frames -> packets -> streams -> HTTP. Most outages are: DNS, routing, firewalls, MTU, or TLS.

Visual stack (OSI-ish)

[L7: HTTP           ]  requests, headers, status codes
|
[L4: TCP/UDP        ]  ports, streams vs datagrams
|
[L3: IP             ]  routing across networks
|
[L2: Ethernet/WiFi  ]  frames inside a LAN (MAC addresses)
|
[L1: Physical       ]  signal on wire/air

Glossary

  • CIDR - prefix length like /24 defining a subnet block
  • ARP - IPv4 neighbor lookup: IP -> MAC on local LAN
  • MTU - max packet size before fragmentation; mismatches cause hangs
  • NAT - address translation (private<->public)
  • stateful firewall - tracks connections; allows return traffic automatically
  • TLS - encryption/authentication on top of TCP (HTTPS)

Visual guides

TCP connect:
client SYN -> server SYN/ACK -> client ACK

Quick triage split:
- DNS?   dig/getent hosts
- Route? ip r / traceroute
- Port?  ss / nc / curl -v
- Wire?  tcpdump

Roadmap core (10, easy -> hard)

  • What's the difference: IP vs TCP?
  • IP routes packets; TCP provides reliable ordered streams on top of IP.
  • What's a subnet (CIDR) in one line?
  • A block of IPs defined by prefix length like /24.
  • What does /24 mean?
  • 24 network bits; 256 addresses total (typically 254 usable in classic IPv4).
  • What's the difference: private vs public IP?
  • Private is non-routable on the public internet; public is internet-routable.
  • What's a default gateway?
  • The router your host uses for non-local destinations.
  • DNS: A vs CNAME vs TXT?
  • A maps name->IPv4; CNAME aliases name->name; TXT stores arbitrary text.
  • What's NAT and why does it exist?
  • Translates addresses (many private -> one public) to conserve IPv4 / simplify.
  • TCP 3-way handshake?
  • SYN -> SYN/ACK -> ACK to establish a connection.
  • How do you troubleshoot "can't reach host" quickly?
  • ip a, ip r, ping, traceroute, dig, ss -tulpn.
  • Explain MTU and a common failure mode.
  • Max frame size; mismatch can cause PMTU blackholes / hangs.

Subnetting & routing (easy -> hard)

  • What's ARP (IPv4) used for?
  • Map IP -> MAC on a local network.
  • Why "same subnet" matters?
  • Same L2 domain; traffic doesn't need a router.
  • What is a route table?
  • Rules mapping destination prefixes to next hops/interfaces.
  • Longest prefix match means?
  • Most-specific route wins (e.g., /32 beats /24).
  • What is asymmetric routing?
  • Forward path differs from return path; can break stateful firewalls/LBs.
  • What's policy-based routing?
  • Routes based on source/marks, not just destination.

DNS in practice (easy -> hard)

  • What is TTL?
  • Cache lifetime; lower TTL speeds changes, increases query load.
  • dig vs getent hosts?
  • dig queries DNS; getent uses system NSS (DNS + hosts + LDAP, etc).
  • Common "DNS works for me" cause?
  • Different resolvers/search domains/caches between hosts/containers.
  • What's split-horizon DNS?
  • Same name resolves differently inside vs outside (private zones).
  • What's negative caching?
  • NXDOMAIN cached; can delay newly created records.

Firewalls, NAT, load balancing (easy -> hard)

  • Stateful vs stateless firewall?
  • Stateful tracks connections; stateless filters each packet.
  • SNAT vs DNAT?
  • SNAT changes source; DNAT changes destination.
  • What is a 4-tuple?
  • srcIP:srcPort -> dstIP:dstPort identifies a flow.
  • L4 vs L7 load balancing?
  • L4 routes TCP/UDP; L7 routes HTTP based on host/path/headers.
  • What's a health check?
  • Probe determining backend availability; drives routing decisions.
  • TLS termination vs passthrough?
  • Termination decrypts at LB; passthrough keeps TLS end-to-end.

Troubleshooting toolkit (easy -> hard)

  • Why curl -v is your friend?
  • Shows DNS, connect, TLS, headers, redirects.
  • ss vs netstat?
  • ss is modern and faster; both show sockets.
  • When to use tcpdump?
  • Prove packet flow, resets, retransmits, DNS answers, MTU issues.
  • How to spot MTU blackhole?
  • Small pings succeed; large fails; tracepath/DF tests reveal.
  • What's "SYN sent" stuck mean?
  • Outbound SYN leaves, no SYN/ACK returns: firewall/routing/ACL/LB.

Sources

  • IETF RFCs, Linux iproute2 docs, vendor docs for your environment.
  • https://www.rfc-editor.org/rfc/rfc793
  • https://www.rfc-editor.org/rfc/rfc791

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