Networking
What to know
Networking questions reward fluency with addressing and ports. Memorize private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), the loopback 127.0.0.1, and APIPA 169.254.0.0/16. An APIPA address means the client could not reach a DHCP server - not that the cable is necessarily bad. Know the common ports: 22 SSH, 23 Telnet, 25/587 SMTP, 53 DNS, 67/68 DHCP, 80 HTTP, 443 HTTPS, 110 POP3, 143 IMAP, 161 SNMP, 445 SMB, and 3389 RDP.
Devices: a hub repeats to all ports, a switch forwards by MAC address, a router forwards between networks by IP, an access point bridges wireless to wired, and a firewall filters traffic. Media: Cat5e/Cat6 twisted pair is limited to 100 m per run; fiber comes in single-mode (long distance) and multimode (shorter, cheaper); plenum cable is required in air-handling spaces. Wireless: 2.4 GHz (802.11b/g/n) travels farther but is crowded - use channels 1, 6, or 11 - while 5 GHz (a/n/ac/ax) is faster over shorter range.
Traps. A host that can ping its gateway and 8.8.8.8 but not www.example.com has a DNS problem; a host that reaches local resources but nothing beyond the gateway has a default gateway or routing problem. Auto-MDI-X makes straight-through versus crossover a legacy distinction. VLANs segment logically but still need a router or Layer 3 switch to communicate.
Rule of thumb. Troubleshoot in order: ping loopback, ping your own IP, ping the gateway, ping a public IP, then resolve a name.
Worked example. A /24 subnet has 256 addresses and 254 usable after the network and broadcast addresses.