Network Diagrams & Troubleshooting

A physical network diagram shows the actual physical arrangement of the components that make up the network, including cables and hardware. It typically gives a bird's-eye view of the network in its physical space — like a floorplan.

Physical topology refers to the physical interconnection of network devices. A physical diagram will reflect one of these arrangements (or a hybrid of them).

Physical diagrams can also capture cabling type/distance, port assignments, and connector types — details that matter when installing or troubleshooting the physical layer.

A logical network diagram describes the way information flows through a network. It typically shows subnets (including VLAN IDs, masks, and addresses), network devices like routers and firewalls, and routing protocols.

Logical topology is how information is actually passed through the network — it's bound to network protocols, as opposed to physical topology, which is just the physical layout. The logical and physical topology of a network may resemble each other, or be entirely different: a twisted-pair Ethernet network is physically a star but follows the bus topology logically. The logical bus is the most common logical topology for wired networks — a sending device broadcasts to the entire network, and other devices check whether the data is addressed to them.

Ethernet is the best-known example of a logical bus topology and is associated with physical bus, star, and tree topologies. Ethernet is contention-based — devices contend (compete) for the right to transmit. All systems listen to the cable, waiting for an opportunity to gain access; if more than one device transmits at once, a collision occurs and both devices wait a random amount of time before retransmitting. This access method is Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

Data flow diagrams (DFDs) are a network representation of a system and the cornerstone of structured systems analysis and design. They use just four symbols to represent a system at any level of detail.

A DFD is analogous to a road map — a network model of all possibilities, with different levels of detail shown at different hierarchical levels. Representing different levels of detail is called leveling (or partitioning): a state road map can be drilled into a city, then into a single street network, without becoming unusable.

Troubleshooting is a methodical process that resolves a problem in the most efficient manner. Often the person solving an issue isn't the person who discovered it, so a structured, repeatable process matters. This same process applies whether you're troubleshooting a switch or a router.

There are several ways to work through isolating a problem across the OSI model. (Note: in the study guide's illustration, the Presentation and Session layers are combined with the Application layer for simplicity.)

Troubleshooting is an art, not a science — there is no single "right" way to troubleshoot everything. With experience, the process becomes automatic and more efficient.