Network Services and Applications
Network Services and Applications
This domain tests the services that make networks usable-DNS, DHCP, NAT, time, and application delivery-and how clients reach them across subnets.
Core rules
- DNS resolves names to records: A/AAAA for addresses, CNAME for aliases, MX for mail, PTR for reverse lookup, TXT for policy and verification. Recursive resolvers query authoritative servers and cache answers by TTL.
- DHCP uses DORA: Discover, Offer, Request, Acknowledge. A relay agent forwards broadcasts to a server on another subnet and sets the gateway address field.
- NAT translates addresses. PAT/overload multiplexes many inside hosts onto one public address using ports. Static NAT maps one-to-one; dynamic NAT uses a pool.
- NTP synchronizes clocks; accurate time is required for certificates, logs, and authentication.
- Load balancers distribute traffic and may terminate TLS. QoS classifies, marks, queues, and drops to protect latency-sensitive traffic.
Worked example Clients on VLAN 20 receive no DHCP address, but the server is reachable by ping. The likely cause is a missing DHCP relay on the VLAN 20 SVI. DHCP Discover is broadcast and routers do not forward broadcasts by default. Adding ip helper-address on the SVI makes the router forward the request with a gateway address, and the server can offer from the correct pool.
Traps Do not confuse an A record with a CNAME; CNAME cannot coexist with other records at the same name. Do not put a DHCP server on every subnet when a relay works. In NAT, inside local/inside global and outside local/outside global are distinct terms. QoS marking is only useful if the trust boundary is set correctly.