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Q5(c) · Longest-prefix matching
2024-12-06 · AI-only Sem V variant
Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.
Q5(c). An 8-bit forwarding table maps prefixes 01→interface1, 10→2, 011→3, 100→4, 001→5, otherwise→6. Determine interfaces for destinations (1)00111101, (2)01010101, (3)11001001 using longest-prefix matching. [5 marks]
Answer
Given
Prefixes01→1,10→2,011→3,100→4,001→5,default→6.
To find
Output interface for each destination.
Formula and why
Among matching prefixes choose the one with the most bits; use default only when none match.
The leading bits must match from the first bit; a substring appearing later does not count.
Solution
Step 1. Compare each destination from the left
| Destination | Matching prefixes | Longest | Interface |
|---|---|---|---|
| 00111101 | 001 | 001 | 5 |
| 01010101 | 01 (but not011) | 01 | 1 |
| 11001001 | None | Default | 6 |
Step 2. Write final order
5, 1, 6.