CompTIA Network+ (N10-009)Networking ConceptsHard
A network engineer wants to summarize the following four contiguous /24 networks into a single route advertisement: 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24. Which single CIDR block correctly summarizes all four networks?
- A192.168.0.0/25
- B192.168.0.0/21
- C192.168.0.0/23
- D192.168.0.0/22
Show answer & explanationAnswer & explanation
Correct answer: D. 192.168.0.0/22
Four contiguous /24 networks (2^2 = 4) can be summarized by borrowing 2 bits from the /24, resulting in a /22. The /22 mask covers addresses from 192.168.0.0 through 192.168.3.255, exactly matching the four /24 subnets.
Why the other options are wrong
- A. A /25 is smaller than a single /24, so it cannot summarize even one full /24 network.
- B. A /21 would summarize eight /24 networks, which is too broad and includes extra unused ranges.
- C. A /23 only summarizes two /24 networks (192.168.0.0-1.255), not all four.
Route Summarization (Supernetting)
Route summarization combines multiple contiguous subnets into a single, larger CIDR block to reduce the number of routes advertised.
- Number of /24s summarized = 2^(24-new prefix)
- 4 x /24 networks combine into one /22
- Summarization reduces routing table size and improves efficiency
Memory trick: 4 blocks of /24 = borrow 2 bits = /22, since 2^2=4.