Network

Understand and troubleshoot computer networks with TCP/IP, DNS, routing, and diagnostic tools.

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Last updated: April 29, 2026

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Manual Installation

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Network Fundamentals

TCP/IP Basics

TCP guarantees delivery with retransmission — use for reliability (HTTP, SSH, databases)
UDP is fire-and-forget — use for speed when loss is acceptable (video, gaming, DNS queries)
Port numbers: 0-1023 privileged (need root), 1024-65535 available — common services have well-known ports
Ephemeral ports for client connections — OS assigns randomly from high range

DNS

DNS resolution is cached at multiple levels — browser, OS, router, ISP — flush all when debugging
TTL determines cache duration — lower before migrations, raise after for performance
A record for IPv4, AAAA for IPv6, CNAME for aliases, MX for mail
CNAME cannot exist at zone apex (root domain) — use A record or provider-specific alias
```
dig
```
and
```
nslookup
```
query DNS directly — bypass local cache for accurate results

IP Addressing

Private ranges: 10.x.x.x, 172.16-31.x.x, 192.168.x.x — not routable on internet
CIDR notation: /24 = 256 IPs, /16 = 65536 IPs — each bit halves or doubles the range
127.0.0.1 is localhost — 0.0.0.0 means all interfaces, not a valid destination
NAT translates private to public IPs — most home/office networks use this
IPv6 eliminates NAT need — but dual-stack with IPv4 still common

Common Ports

22: SSH — 80: HTTP — 443: HTTPS — 53: DNS
25/465/587: SMTP (mail sending) — 143/993: IMAP — 110/995: POP3
3306: MySQL — 5432: PostgreSQL — 6379: Redis — 27017: MongoDB
3000/8080/8000: Common development servers

Troubleshooting Tools

```
ping
```
tests reachability — but ICMP may be blocked, no response doesn't mean down
```
traceroute
```
/
```
tracert
```
shows path — identifies where packets stop or slow down
```
netstat -tulpn
```
or
```
ss -tulpn
```
shows listening ports — find what's using a port
```
curl -v
```
shows full HTTP transaction — headers, timing, TLS negotiation
```
tcpdump
```
and Wireshark capture packets — last resort for deep debugging

Firewalls and NAT

Stateful firewalls track connections — allow response to outbound requests automatically
Port forwarding maps external port to internal IP:port — required to expose services behind NAT
Hairpin NAT for internal access to external IP — not all routers support it
UPnP auto-configures port forwarding — convenient but security risk, disable on servers

Load Balancing

Round-robin distributes sequentially — simple but ignores server capacity
Least connections sends to least busy — better for varying request durations
Health checks remove dead servers — configure appropriate intervals and thresholds
Sticky sessions (affinity) keep user on same server — needed for stateful apps, breaks scaling

VPNs and Tunnels

VPN encrypts traffic to exit point — all traffic appears from VPN server IP
Split tunneling sends only some traffic through VPN — reduces latency for local resources
WireGuard is modern and fast — simpler than OpenVPN, better performance
SSH tunnels for ad-hoc port forwarding —
```
ssh -L local:remote:port
```
creates secure tunnel

SSL/TLS

TLS 1.2 minimum, prefer 1.3 — older versions have known vulnerabilities
Certificate chain: leaf → intermediate → root — missing intermediate causes validation failures
SNI allows multiple certs on one IP — older clients without SNI get default cert
Let's Encrypt certs expire in 90 days — automate renewal or face outages

Common Mistakes

Assuming DNS changes are instant — TTL means old records persist in caches
Blocking ICMP entirely — breaks path MTU discovery, causes mysterious failures
Forgetting IPv6 — services may be accessible on IPv6 even with IPv4 firewall
Hardcoding IPs instead of hostnames — breaks when IPs change
Not checking both TCP and UDP — some services need UDP (DNS, VPN, game servers)
Confusing latency and bandwidth — high bandwidth doesn't mean low latency

Network

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Quick Start

Manual Installation

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Network Fundamentals

TCP/IP Basics

DNS

IP Addressing

Common Ports

Troubleshooting Tools

Firewalls and NAT

Load Balancing

VPNs and Tunnels

SSL/TLS

Common Mistakes

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