CCIE Security (legacy) — Question 4

A router has four interfaces addressed as 10.1.1.1/24, 10.1.2.1/24, 10.1.3.1/24, and 10.1.4.1/24. What is the smallest summary route that can be advertised covering these four subnets?

Answer options

• A. 10.1.2.0/22
• B. 10.1.0.0/22
• C. 10.1.0.0/21
• D. 10.1.0.0/16

Correct answer: C

Explanation

The correct answer is C, 10.1.0.0/21, as it covers the IP addresses from 10.1.0.0 to 10.1.7.255, which includes all the specified subnets. Option A (10.1.2.0/22) only covers 10.1.2.0 to 10.1.3.255, missing the first and last subnet. Option B (10.1.0.0/22) covers from 10.1.0.0 to 10.1.3.255, which also excludes the last subnet. Option D (10.1.0.0/16) is too broad and not the smallest summary route.