Consider the following routing table at an IP router: \(\begin{array}{|l|l|l|}…
2015
Consider the following routing table at an IP router:
\(\begin{array}{|l|l|l|} \hline \textbf {Network No} & \textbf {Net Mask} & \textbf{Next Hop} \\\hline \text {128.96.170.0} & \text{255.255.254.0} & \text{Interface $0$} \\\hline\text {128.96.168.0} & \text{255.255.254.0} & \text{Interface $1$} \\\hline\text {128.96.166.0} & \text{255.255.254.0} & \text{R$2$}\\\hline \text {128.96.164.0} & \text{255.255.252.0} & \text{R$3$}\\\hline \text {0.0.0.0} & \text{Default} & \text{R$4$}\\\hline \end{array}\)
For each IP address in Group I identify the correct choice of the next hop from Group II using the entries from the routing table above.
Group I
i) 128.96.171.92
ii) 128.96.167.151
iii) 128.96.163.151
iv) 128.96.165.121
Group II
a) Interface 0
b) Interface 1
c) R2
d) R3
e) R4
- A.
i-a, ii-c, iii-e, iv-d
- B.
i-a, ii-d, iii-b, iv-e
- C.
i-b, ii-c, iii-d, iv-e
- D.
i-b, ii-c, iii-e, iv-d
Attempted by 110 students.
Show answer & explanation
Correct answer: A
Key idea: use longest-prefix (most specific) match among the routing entries; if none match, use the default route.
128.96.171.92 → matches 128.96.170.0 with mask 255.255.254.0 (/23 covers 128.96.170.0–128.96.171.255): next hop Interface 0.
128.96.167.151 → matches both 128.96.166.0/23 and 128.96.164.0/22, but /23 is longer (more specific): next hop R2.
128.96.163.151 → does not match any specific listed network, so use the default route: next hop R4.
128.96.165.121 → matches 128.96.164.0 with mask 255.255.252.0 (/22 covers 128.96.164.0–128.96.167.255): next hop R3.
Final mapping: 128.96.171.92 → Interface 0; 128.96.167.151 → R2; 128.96.163.151 → R4; 128.96.165.121 → R3.