A compact reference for addressing, routing, switching, services, wireless, operations, and troubleshooting. Use it to reinforce understanding, not to replace lab work or the official objectives.
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Treat these as common defaults, not proof that a service must use that port in every environment.
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Check the reviewed Network+ exam recordUse this sheet to reinforce a troubleshooting model, not to memorize isolated trivia. Begin every scenario by defining scope: one device, one subnet, one floor, one site, one service, or everyone. Scope immediately narrows the likely fault domain and determines whether you should start at the endpoint, access layer, distribution/core path, WAN edge, name-resolution service, or application.
For physical-layer problems, look for link state, cabling category, connector condition, transceiver compatibility, signal strength, interference, distance limits, power delivery, and interface errors. A down link or rapidly increasing physical errors should be investigated before spending time on routing protocols or DNS. Always choose the lowest-layer plausible test first when evidence points there.
At Layer 2, focus on VLAN membership, trunk configuration, native VLAN mismatch, spanning-tree state, MAC learning, port security, loops, and duplex or speed negotiation. A host may have a valid IP address yet still fail locally if its switch port is in the wrong VLAN or the trunk does not carry that VLAN toward the gateway.
At Layer 3, check address, prefix, gateway, route table, next hop, ACLs, NAT behavior, and dynamic-routing adjacency. If local subnet communication succeeds but remote networks fail, the default gateway and routing path deserve attention. If only one destination fails, trace the path and compare routes rather than assuming the entire WAN is broken.
DHCP failures can leave a host with an automatic private address or no usable gateway. DNS failures can make applications appear offline even when IP connectivity works. NTP drift can break authentication and certificates. AAA or directory failures can block network access despite healthy links. Proxy, VPN, and firewall policies can also create selective failures that users describe simply as “the network is down.”
Remember the relationship between TCP and UDP rather than only port numbers. TCP provides connection-oriented delivery, ordering, and retransmission; UDP minimizes overhead and is common for real-time or request/response protocols where the application handles loss differently. Port memorization is useful, but protocol behavior often explains why a troubleshooting symptom looks the way it does.
Wireless troubleshooting requires both RF and network thinking. Interference, channel overlap, attenuation, low signal-to-noise ratio, excessive client density, roaming behavior, authentication failure, and upstream congestion can all produce slow or unreliable service. Compare signal quality with latency, retransmission, and throughput rather than treating signal bars as a complete diagnosis.
Latency is delay, jitter is variation in delay, packet loss is dropped traffic, and bandwidth is capacity. Throughput is the actual data rate achieved. A high-bandwidth path can still perform poorly for voice if latency and jitter are excessive. A low-latency path can still be constrained by congestion if available capacity is insufficient for the offered load.
For IPv4 subnetting, know the common host counts and block sizes for /24 through /30. Remember that usable-host calculations normally subtract network and broadcast addresses in traditional subnets. Identify the interesting octet, calculate the block size, then find the network boundary below the target address. Consistent method beats mental guessing under exam pressure.
For IPv6, recognize link-local, global unicast, multicast, loopback, and common transition concepts. IPv6 does not use broadcast in the IPv4 sense. Router advertisements support neighbor discovery and SLAAC behavior, while DHCPv6 can provide addressing or additional configuration depending on design. Focus on how the mechanism is used rather than memorizing every possible prefix.
When asked for the FIRST troubleshooting step, do not jump directly to replacing equipment or changing production configuration unless the evidence clearly justifies it. Confirm the problem, establish a theory, test the theory, plan the fix, implement it, verify full functionality, and document results. The exam frequently rewards process discipline.
If two tools could help, choose the one that directly tests the suspected layer. Use interface statistics and cable testing for physical problems, ARP/MAC information for local adjacency, ping for reachability, traceroute for path analysis, DNS tools for resolution, packet capture for protocol detail, and log or monitoring systems for historical correlation.
End your review with your personal confusion list: ports you swap, routing concepts you reverse, wireless standards you forget, subnetting mistakes you repeat, and troubleshooting steps you skip. Reviewing those patterns immediately before the exam is usually more valuable than rereading every objective from the beginning.