Four supporting services every production network depends on: DNS resolves names to IPs, NTP keeps clocks accurate, SNMP polls device health, and syslog records events centrally for forensics.
Hosts resolve www.example.com to an IP via DNS (UDP/TCP port 53). Routers can be configured with name servers so that 'ping example.com' works from the CLI. In production, recursive resolvers handle clients; routers should only resolve names for their own administrative needs.
R1(config)# ip name-server 8.8.8.8 1.1.1.1
R1(config)# ip domain-lookupNTP keeps device clocks within milliseconds of a reference. Correlating logs across devices during an incident is impossible if their clocks disagree by minutes. Use a strict hierarchy: small devices sync from one or two stratum-2/3 servers, never directly from public stratum-1.
R1(config)# ntp server 0.pool.ntp.org
R1(config)# ntp server 1.pool.ntp.org
R1# show ntp status2 more sections in this chapter, plus a 3-question quiz.
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