Domain 3 - Networking, Infrastructure, and Internet Services
This domain tests LAN and WLAN design, IP addressing, segmentation, filtering, bandwidth planning, and monitoring. Items are usually quantitative: you are expected to size a circuit, a scope, or a power budget from stated numbers.
Core rules and concepts
- A /24 subnet yields 254 usable host addresses. Size DHCP scopes above the device count on that VLAN; a 1:1 rollout frequently exhausts a scope sized for the old lab.
- Segment with VLANs: staff, student, guest, IoT/AV, and management traffic should not share a broadcast domain or a filter policy.
- Wireless: reuse channels (1/6/11 on 2.4 GHz; non-overlapping 5 GHz channels), design for capacity rather than coverage, and confirm PoE budget - twenty 30 W access points need at least 600 W of switch PoE.
- Copper runs are limited to 100 m; use fiber for building-to-building backbone. Compare firewall throughput under threat inspection, not the marketing number.
Worked example
A school has 1,200 students with two devices each, 2,400 devices total. Assume 60% concurrent, about 1,440 active devices, planned at 2 Mbps each: roughly 2.9 Gbps of aggregate demand. A 1 Gbps circuit is therefore under-provisioned, and the internal uplink must exceed the WAN so the firewall is not the bottleneck. Recommendation: a multi-gigabit internet circuit with 10 Gbps backbone uplinks, verified against measured firewall throughput with filtering enabled.
Traps that cost marks
Confusing Mbps and Gbps in sizing answers; ignoring protocol overhead and peak concurrency; adding access points without channel reuse; forgetting that the content filter and firewall both consume throughput.