network

name	Network
description	Understand and troubleshoot computer networks with TCP/IP, DNS, routing, and diagnostic tools.
metadata	{"clawdbot":{"emoji":"🌐","os":["linux","darwin","win32"]}}

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