National Legal Aid Services Organization — Assistant Maintenance Engineer — Exam: 18.10.2025প্রশ্ন ও সমাধান

Feature	Star Topology	Mesh Topology
Structure	All nodes connect to one central hub/switch	Every node connects directly to every other node
Links needed (n nodes)	n	n(n−1)/2
Cost	Lower (fewer cables)	Higher (many redundant cables)
Fault tolerance	Single point of failure — hub failure downs the whole network	Highly fault-tolerant — multiple alternate paths
Ease of expansion	Easy — add a new spoke to the hub	Complex — every new node needs links to all existing nodes
Typical use	LANs, home/office networks	Backbone/critical networks (ISP cores, mission-critical links)

graph TD
    subgraph Star
    H((Hub)) --- A1[Node A]
    H --- A2[Node B]
    H --- A3[Node C]
    H --- A4[Node D]
    end

graph LR
    subgraph Mesh
    B1[Node A] --- B2[Node B]
    B1 --- B3[Node C]
    B1 --- B4[Node D]
    B2 --- B3
    B2 --- B4
    B3 --- B4
    end

• Formula check for 4 nodes: Star needs 4 links (1 per node to hub); Mesh needs 4(4-1)/2 = 6 links — visibly denser and costlier, but with no single point of failure since traffic can route around any single broken link.

Exam takeaway: Star = n links, cheap, single point of failure (the hub); Mesh = n(n−1)/2 links, expensive, but maximum redundancy/fault-tolerance.

A Packet-Filtering Firewall is the most basic type of firewall — it inspects each packet's header (source/destination IP, port numbers, protocol) against a set of predefined rules, and either allows or blocks it, without looking at the packet's payload/content or tracking connection state.

• Operates at: Network layer (Layer 3) and Transport layer (Layer 4) of the OSI model.
• Decision basis: a static rule list (Access Control List / ACL), e.g.:

Rule #	Source IP	Dest Port	Protocol	Action
1	Any	80, 443	TCP	Allow
2	Any	23	TCP	Deny (Telnet blocked)
3	192.168.1.0/24	Any	Any	Allow
4	Any	Any	Any	Deny (default)

flowchart LR
    P[Incoming Packet] --> H{Match a rule?}
    H -- Allow rule --> A([Forward packet])
    H -- Deny rule --> B([Drop packet])
    H -- No match --> C([Default deny])

Strengths: fast, low overhead, simple to configure.
Weaknesses: stateless — it examines each packet in isolation, with no memory of prior packets in the same connection, so it cannot detect spoofed packets pretending to belong to an established session, and it can't inspect payload content (application-layer attacks pass through undetected). This is why modern firewalls use stateful inspection or Next-Gen Firewalls (NGFW) that also track connection state and inspect payloads.

Exam takeaway: Packet filter = rule-based, stateless, header-only inspection at Layers 3-4 — fast but blind to connection state and payload content.