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

Behind one line of an automation script, requests.get("https://api.example.com/devices"), DNS must resolve the name, NAT must translate the source address, and it must reach the target on the right port number (443 for HTTPS). If any one of these IP services or ports is missing, the symptom looks like "the app is broken," yet the causes are entirely different. This section organizes the roles of DHCP, DNS, NAT, SNMP, and NTP that apps/automation depend on, plus the must-memorize key port numbers, as knowledge for pinpointing "which service/port this failure belongs to."

6.3.1Key IP services

• DHCP (UDP 67 server / 68 client) auto-distributes IP address, subnet mask, default gateway, and DNS server to a host. If DHCP fails, the host lacks a correct IP/GW/DNS in the first place, so no downstream communication succeeds.
• DNS (UDP/TCP 53) resolves a hostname to an IP address. Since automation specifies endpoints by FQDN, if DNS cannot resolve, it fails with Name or service not known before connecting. NAT translates private IPs to global (and vice versa) so internal hosts can talk to the outside; if the return mapping (translation table) breaks, symptoms like one-way-only reachability appear.
• SNMP (UDP 161, traps 162) monitors device state by polling/notification. NTP (UDP 123) synchronizes device/server time. Time skew is fatal to automation because TLS certificate validity, API token expiry, log correlation, and Kerberos auth are time-dependent, so bad NTP often surfaces as "certificate error" or "auth failure."