IP Services
The IP Services domain (10%) covers the supporting services that make a network usable: DHCP, DNS, NAT, NTP, SNMP, syslog, and QoS. Questions tend to be about message flows, address translation terms, and severity or stratum values.
DHCP and DNS
DHCP uses DORA: Discover, Offer, Request, Acknowledge. A relay agent (ip helper-address) forwards broadcasts to a server on another subnet. DNS resolves names; A records map IPv4, AAAA map IPv6, and CNAME creates an alias. DHCPv6 uses solicit/advertise/request/reply and can operate statelessly with SLAAC.
NAT
Static NAT maps one inside local address to one inside global address. PAT, also called overload, multiplexes many inside hosts using source ports. Terms: inside local is the host's pre-translation address; inside global is its translated address; outside global is the destination's real address; outside local is the destination as seen from inside. NAT is applied after routing for outbound traffic.
NTP, SNMP, and syslog
NTP stratum 1 is directly attached to a reference clock; higher strata are further away. SNMPv3 adds authentication and encryption; v2c uses community strings. Syslog severities run 0–7: 0 emergency, 1 alert, 2 critical, 3 error, 4 warning, 5 notice, 6 informational, and 7 debugging. Lower numbers are more severe.
QoS
QoS classifies and marks traffic using DSCP at Layer 3 and CoS at Layer 2, then polices or shapes, queues, and avoids congestion. Voice needs low latency, jitter, and loss; prioritize it with LLQ or priority queuing.
Worked example
Host 10.1.1.10:5000 sends to 8.8.8.8:53. The NAT router translates the source to 203.0.113.5:40000. Inside local = 10.1.1.10:5000; inside global = 203.0.113.5:40000; outside global = 8.8.8.8:53. A reply is destined first to 203.0.113.5:40000.
Traps
Reversing inside and outside terms; thinking PAT requires a pool; mixing up syslog level 3 (error) with level 6 (informational); and treating NTP stratum 0 as valid.