What is route summarization? Summarize 192.168.0.0/24 and 192.168.1.0/24.
Summarization replaces several contiguous routes with one shorter-prefix route. 192.168.0.0/24 and 192.168.1.0/24 combine into 192.168.0.0/23, which covers 192.168.0.0 to 192.168.1.255, so routers keep one entry instead of two.
Explain it simply
Instead of telling the courier about House 1 and House 2 separately, you say "everything on Lane A". Fewer entries to remember, same delivery.
Example
192.168.0.0/24 = 192.168.00000000.0
192.168.1.0/24 = 192.168.00000001.0
Common leading bits: 23 -> 192.168.0.0/23
Four networks 192.168.0.0 to 192.168.3.0 (/24 each):
third octet 0,1,2,3 = 000000xx -> 22 common bits -> 192.168.0.0/22
Not summarizable: 192.168.1.0/24 + 192.168.2.0/24
third octet 00000001 and 00000010 -> differ early,
/23 blocks start at even numbers (0, 2, 4) so 1.0 is not aligned.Two conditions: the networks must be contiguous, and the first one must start on a boundary that fits the new, larger block size. Summarization keeps routing tables small, and is how ISPs advertise large blocks.
What to say in the interview
"Count the common leading bits across the networks. Two adjacent /24s that start on an even third octet merge into a /23. The blocks must be contiguous and aligned, otherwise the summary would include addresses that do not belong."
Common mistakes
- Merging 1.0/24 and 2.0/24 into a /23. They are adjacent but not aligned.