Networking cheatsheet

A one-page reference for networking fundamentals. For the OSI/TCP-IP models, subnetting, and TLS handshake detail, see the complete guide.

📖 Full guide: Networking →

Troubleshooting tools

ping -c 4 example.com
traceroute example.com
mtr example.com                          # continuous traceroute+ping
curl -v https://example.com               # full request/response + TLS
dig example.com                            # DNS resolution
tcpdump -i eth0 port 443
ss -tulpn                                   # listening/established sockets
nc -zv example.com 443                       # port reachability

Diagnostic order under pressure

1. ping — is the path up? (ICMP is often blocked, so a failed ping ≠ down)
2. dig/nslookup — does the name resolve to the expected IP?
3. nc -zv / curl — is the port/service accepting connections?
4. traceroute/mtr — where is latency/loss happening?
5. tcpdump — inspect actual bytes on the wire.

TCP vs UDP

	TCP	UDP
Connection	3-way handshake	connectionless
Reliability	guaranteed, ordered	best-effort
Use case	HTTP, DB conns	DNS queries, video, gaming

DNS resolution flow

Browser cache → OS cache → resolver (recursive) →
root → TLD → authoritative nameserver → answer

L4 vs L7 load balancing

• L4 — routes on IP/port, fast, protocol-agnostic.
• L7 — routes on HTTP content (headers, path, host) — enables path-based routing, TLS termination, content-aware rules.

CIDR quick reference

/32 = 1 host        /24 = 256 addrs (typical subnet)
/16 = 65,536 addrs  /8  = 16.7M addrs

Firewalls & NAT

NAT translates private IPs to a public one at the network edge; firewalls filter by IP/port/protocol (and L7 rules for app firewalls). A "connection refused" often means firewall/security-group, not a dead service.

See: Common Failure Scenarios an SRE Debugs