Functions of OSI and TCP/IP Layers Q.28
28An IP router implementing Classless Interdomain Routing (CIDR) receives a packet with address 131.23.151.76. The router’s routing table has the following entries:
| Prefix | Output Interface Identifier |
| 131.16.0.0 / 12 | 3 |
| 131.28.0.0 / 14 | 5 |
| 131.19.0.0 / 16 | 2 |
| 131.22.0.0 /15 | 1 |
The identifier of the output interface on which this packet will be forwarded is_______.
- Option : A
- Explanation : Given address 131.23. 151.76. coming to the first field of given routing table
⟹ 131.16. 0.0/12
131.0001 0111.151.76
131.0001 0000.0.0 ( given mask bits = 12)
⟹ 131.16.0.0 Matched
Coming to the 2nd field of given Routing table
⟹ 131.28.0.0/14
131.0001 0111. 151. 76
131.0001 0100.0.0 ( given mask bits = 14)
⟹ 131.20.0.0 not matched
Coming to the 3rd field of given Routing table
Error ! Not a valid link 131.19.0.0/16
131.0001 0111.151.76
131.0001 0111.0.0 ( given mask bits = 16)
⟹ 131.23.0.0 Not matched
Coming to the 4th field of given Routing table
⟹ 131.22.0.0/15
131.0001 0111.151.76
131.0001 0110.0.0 ( given mask bits = 15)
⟹ 131.22.0.0 Matched
We are getting 1st and 4th entries are matched so among them we have to picked up the longest mask bit, so output interface identifier is 1.
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