DHCP, DNS, and NTP Basics

Watch first

Plain-English explanation

DHCP assigns IP parameters automatically — address, mask, gateway, DNS servers, lease time. Clients broadcast DISCOVER; servers offer OFFER; client REQUESTS; server ACKs — DORA.

DNS maps names to IPs — clients query recursive resolvers; resolvers traverse hierarchy (root → TLD → authoritative). Browsers need DNS even when routing works.

NTP synchronizes clocks — critical for correlated logs, certificate validity, and OSPF authentication timestamps.

Deep dive

DHCP relay (ip helper-address): Forwards broadcasts to remote DHCP server on another subnet — router converts broadcast to unicast toward server.

DNS record types (high-yield):

| Type | Purpose | |------|---------| | A | Name → IPv4 | | AAAA | Name → IPv6 | | CNAME | Alias | | MX | Mail server |

SNMP/Syslog (awareness): SNMP polls/collects metrics; Syslog sends events to collectors — both appear in CCNA services domain lightly.

Step-by-step — Cisco DHCP pool

ip dhcp pool LAN10
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1
 dns-server 8.8.8.8
 lease 7
!
interface GigabitEthernet0/0
 ip address 192.168.10.1 255.255.255.0
!
ip dhcp excluded-address 192.168.10.1 192.168.10.10

Client receives .11+ with gateway .1 and DNS 8.8.8.8.

Relay example:

interface GigabitEthernet0/1
 ip address 10.10.20.1 255.255.255.0
 ip helper-address 192.168.100.50

Commands to know

show ip dhcp binding
show ip dhcp pool
debug ip dhcp server packet

ntp server 203.0.113.1
show ntp status
show clock

Troubleshooting

| Symptom | Check | |---------|-------| | APIPA 169.254.x.x | No DHCP response — server down, wrong VLAN, no relay | | IP but no Internet | Missing default-router option or DNS | | Works locally, names fail | DNS server option wrong or firewall on UDP/TCP 53 | | Log timestamps wrong | NTP not synced — show ntp associations |

DHCP problems are often L2/VLAN or missing helper-address, not exotic server bugs.

Exam relevance

Practice checklist

• Draw DORA sequence with broadcast vs unicast steps
• Configure DHCP pool with exclusions on lab router
• Add helper-address on a remote subnet and verify lease
• Explain why NTP matters for troubleshooting two routers
• Resolve a hostname to IP using nslookup or dig on a PC