Network and Infrastructure Operations
Domain 3 tests device roles, subnetting arithmetic, cabling and room limits, and layered fault isolation. Expect one subnetting question and one troubleshooting-sequence question.
Devices and addressing
A hub repeats to all ports (one collision domain); a switch forwards by MAC within a broadcast domain; a router forwards by IP between networks; a firewall filters by policy. VLANs split one switch into separate broadcast domains and need routing or a Layer 3 switch to communicate. NAT translates private to public addresses. Static routes suit small stable networks; dynamic routing suits changing topologies.
Cabling and environment
Copper twisted pair (Cat5e, Cat6) is limited to 100 m per run; 1000BASE-T runs on Cat5e, 10GBASE-T needs Cat6/6a and shorter distances. Fibre suits long runs and electromagnetic interference; single-mode for distance, multimode for building backbones. Label both ends of every patch lead and record the port in the configuration documentation. The equipment room needs UPS and dual power feeds, temperature around 18–27 °C, humidity roughly 40–60%, grounding and anti-static precautions.
Troubleshooting by layer
Work physical first (power, link light, cable, port), then data link (MAC, VLAN membership, duplex mismatch), then network (IP, mask, gateway, route), then transport (port open, firewall rule), then application. Tools: ipconfig/ifconfig, ping, arp, traceroute, nslookup, netstat, a port test with telnet or curl, and packet capture when the evidence is contradictory.
Worked example
Host 192.168.10.70/26. A /26 mask is 255.255.255.192, block size 256 − 192 = 64, so subnets begin at .0, .64, .128, .192. Address .70 falls in 192.168.10.64/26: network .64, broadcast .127, usable range .65–.126. Reasoning: divide the host octet by the block size; the floor multiple is the network address.
Traps
- A failed ping does not prove the host is down; ICMP is often filtered.
- Confusing /26 with /24 and mis-stating usable hosts (62, not 254).
- A duplex mismatch gives slow, error-prone traffic rather than a dead link.
- Cable runs over 100 m cause intermittent CRC errors that look like application faults.
- Testing only from the server side and ignoring the client path.