Networking: a practical CompTIA A+ guide
Addressing, routing, Wi-Fi, ports, and command-line diagnosis. Learn the mental models, common failure modes, and evidence-based troubleshooting steps.
01. Follow the packet
Separate link, local network, name resolution, and internet routing. An unlit Ethernet link LED suggests a physical connection problem: test cable, NIC and switch port. A 169.254.x.x address often means IPv4 DHCP failed and the client self-assigned APIPA. A valid local IP with wrong default gateway can reach the local subnet but not other networks. If you can ping an IP but not its hostname, investigate DNS.
02. Addressing and subnet basics
Private IPv4 space includes 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. A /24 mask has 256 addresses in its block; in an ordinary subnet, 254 are usable hosts after network and broadcast. The gateway must be reachable from the client subnet. DHCP typically provides address, mask, gateway and DNS. DNS does not assign IP addresses; it translates names.
03. Ports in context
HTTP 80 and HTTPS 443 carry web traffic. SSH 22 provides encrypted remote terminal access; Telnet 23 is unencrypted. DNS typically uses UDP 53 and may use TCP 53. DHCP uses UDP 67/68. SMB uses TCP 445 and RDP typically TCP 3389. Do not diagnose an application solely from one port: confirm service state, firewall rules, endpoint, and route.
04. Wi-Fi diagnosis
2.4 GHz often reaches farther through obstacles but is more prone to interference; 5/6 GHz can offer more capacity at shorter range. Examine signal strength, channel overlap, band support, channel width, security mode, and client density. A correct password does not guarantee a usable RF link. WPA3 is preferred when supported; avoid WEP and legacy TKIP.
05. Windows field toolkit
Use ipconfig /all to inspect configuration, ping for reachability, tracert for route hops, nslookup for DNS lookups and netstat for connections. Ping alone does not prove an app works because ICMP may be filtered. Change one item, re-test, and document evidence.
Apply the lesson: real support cases
A printer at 192.168.50.20/24 can print locally but cannot reach a server on 10.20.0.0/16. Check its default gateway and routing before resetting the printer.
Only one meeting-room laptop loses Wi-Fi when the microwave runs. Compare 2.4 GHz vs 5 GHz, RSSI and interference rather than replacing the company firewall.
These are original explanatory notes aligned to the named exam domains; review the current official objectives for exact scope. The mock exam is practice only and is not scored on CompTIA's scale.