A network engineer is implementing route summarization in an OSPF network. The goal is to advertise a single summary route for the following networks: 192.168.16.0/24, 192.168.17.0/24, 192.168.18.0/24, and 192.168.19.0/24. Which is the most efficient summary route that includes all these networks and no unnecessary addresses?
- A192.168.0.0/16
- B192.168.16.0/22
- C192.168.16.0/20
- D192.168.16.0/21
Show answer & explanationAnswer & explanation
Correct answer: B. 192.168.16.0/22
To find the most efficient summary, convert the network addresses to binary and find the longest common prefix. The networks are 192.168.16.0, 192.168.17.0, 192.168.18.0, and 192.168.19.0. The first two octets (192.168) are common (16 bits). For the third octet: 16 = 00010000 17 = 00010001 18 = 00010010 19 = 00010011 The common prefix for the third octet is '000100' (6 bits). So, the total common prefix is 16 (from 192.168) + 6 (from 16-19) = 22 bits. The summary network address is 192.168.16.0/22.
Why the other options are wrong
- A. 192.168.0.0/16 includes too many unnecessary addresses, making it inefficient.
- C. 192.168.16.0/20 is too broad; it covers 192.168.16.0 - 192.168.31.255.
- D. 192.168.16.0/21 is also too broad; it covers 192.168.16.0 - 192.168.23.255.
Route Summarization (CIDR)
Route summarization, or Classless Inter-Domain Routing (CIDR), combines multiple contiguous IP network addresses into a single, more general address to reduce routing table size.
- Reduces routing table entries.
- Improves routing efficiency and stability.
- Calculated by finding the longest common binary prefix.
Memory trick: Binary Conversion Finds Common Prefix for Consolidation.