6Network Fundamentals

• 6.1L2/L3 fundamentals (MAC, VLAN, IP, subnet, gateway)

  As the prerequisite for an automation script or API client to actually reach its target, this section covers L2 MAC addresses and VLANs, and L3 IP addresses, subnet mask/prefix, routing, and the default gateway—framed as the connectivity diagnosis of "why can only this host not reach the API."

• 6.2Components and topology (switch, router, firewall, LB, and the three planes)

  Covers the roles of the switch, router, firewall, and load balancer (LB); reading topology diagrams and interface/port values; and the view that splits a device into the management plane, control plane, and data plane—framed as the judgment of "which plane automation touches and which device along the path can stop an API call."

• 6.3IP services and port numbers (DHCP, DNS, NAT, SNMP, NTP)

  Covers the common IP services apps/automation depend on—DHCP (auto IP assignment), DNS (name resolution), NAT (address translation), SNMP (monitoring), NTP (time sync)—and the port numbers to memorize (SSH 22 / Telnet 23 / HTTP 80 / HTTPS 443 / NETCONF 830 / SNMP 161 / NTP 123 / DNS 53 / DHCP 67-68), framed as the diagnosis of "which missing service breaks an API call and how."

• 6.4Diagnosing app connectivity (NAT, port blocking, proxy, VPN)

  Systematically isolates app/automation connection failures as NAT translation issues, port blocking by firewalls, the need to traverse a corporate proxy, VPN connection/split-tunnel behavior, and the effect of network constraints such as latency/MTU/rate limits on apps. Builds the diagnostic skill of linking "error symptom -> the network factor to suspect -> the verification command."