Network Types & Topologies

Network Classifications

Network "type" on the exam is about geographic scope, not speed. Match the scope word to the example.

Trap: a SAN is not the same as NAS. A SAN is a dedicated high-speed network that presents block storage to servers; NAS is a single file-serving appliance on the regular LAN.

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Network Topologies

CompTIA distinguishes the physical layout (how cables run) from the logical flow (how data moves). Know each topology's failure behavior.

Star (most common today)

• Every device homes to a central switch.
• One device or cable failing affects only that node.
• The central switch is a single point of failure - if it dies, the whole segment drops.
• Easy to expand and troubleshoot (isolate one port at a time).

Mesh

• Full mesh: every node links to every other node. A network of n nodes needs n(n-1)/2 links - so 6 nodes need 15 cables.
• Partial mesh: nodes link to several, not all, peers.
• Maximum redundancy: traffic reroutes around any single failed link.
• Costly to cable; used for WAN backbones, critical infrastructure, and wireless mesh.

Bus (legacy)

• All devices tap one shared backbone cable.
• A terminator at each end stops signal reflection.
• A single break or missing terminator can take down the whole segment; collision-prone and slow.

Ring (legacy)

• Devices form a closed loop; data circulates in one direction (dual-ring adds failover).
• A single break halts the ring unless a redundant path exists. Seen in legacy Token Ring and FDDI.

Hybrid

• A blend of types - most real enterprises run star-of-stars wiring with mesh links between core switches for redundancy.

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VLANs (Virtual LANs)

A VLAN logically slices one physical switch into separate broadcast domains.

Sample design:

Key rule: devices on different VLANs cannot communicate without a router or Layer 3 switch performing inter-VLAN routing. That enforced separation is exactly why VLANs improve security.

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Supporting Services

DHCP relay

Because routers do not forward broadcasts, a client on a subnet with no local DHCP server would never reach one. A DHCP relay agent (often configured on the router as an IP helper) catches the client's broadcast and forwards it as unicast to the central DHCP server on another subnet.

DNS resolution hierarchy

A name resolves top-down: Root servers -> TLD servers (.com, .org) -> authoritative servers for the domain. The client checks its local cache, then the router/ISP resolver, before walking the hierarchy.

• Recursive query: the resolver does all the legwork and returns a final answer.
• Iterative query: each server hands back a referral to the next server to ask.

Understanding both flows explains why ipconfig /flushdns (local cache) and waiting for TTL expiry (downstream caches) are both sometimes needed after a DNS change.

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Physical vs. Logical Topology

A topology question can describe the same network two different ways, so the exam separates the physical topology (how the cables actually run) from the logical topology (the path data follows). Classic Ethernet over a hub was wired as a physical star - every device cabled back to the hub - yet behaved as a logical bus, because the hub flooded every signal to all ports and all devices shared one collision domain. Modern switched Ethernet is a physical star that also behaves as a logical star, since the switch forwards each frame only toward its destination.

Recognizing that the wiring shape and the data-flow shape can differ keeps you from picking the wrong answer when a question emphasizes one over the other.

Why Star Won and Mesh Persists

Star topology dominates because it isolates faults: a bad cable or NIC affects only its own port, and adding a device means running one cable to the switch. Its weakness is the central switch as a single point of failure, which enterprises mitigate with redundant switches and stacked uplinks. Mesh survives in places where uptime outweighs cost - WAN backbones, data-center spine-leaf fabrics, and self-healing wireless mesh systems - because its many redundant links let traffic reroute instantly around a failure.

The cabling cost is the catch: a full mesh of just 8 nodes needs 28 links, which is why true full mesh is rare outside the network core.

A Segmentation Scenario

A clinic wants its medical devices, staff laptops, and patient guest Wi-Fi kept apart on one physical switch. The correct design uses VLANs: one VLAN per group, each in its own subnet and broadcast domain, with a router or Layer 3 switch performing inter-VLAN routing only where policy permits - for example, letting staff reach a print server while blocking guests entirely. If the DHCP server lives on the management VLAN, each other VLAN needs a DHCP relay (IP helper) so client broadcasts reach it across the router boundary.

The exam-worthy takeaway is that VLANs deliver department-level isolation without buying a separate switch for each group, and that crossing a VLAN boundary always requires a routing function.