Lesson 6 of 7
Summarising Routes
A summary route advertises many networks as one. It keeps routing tables small and stable, as long as the networks line up.
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Worked Example: Four /24s That Line Up
10.53.8.0/24
,
10.53.9.0/24
,
10.53.10.0/24
,
10.53.11.0/24
.
Four networks is 22, so the summary is 2 bits shorter than /24: a /22. A /22 moves in blocks of 4 in the third octet, and 8 is a multiple of 4, so the four networks fill exactly one block:
10.53.8.0/22
Worked Example: Four /24s That Do Not
10.53.9.0/24
,
10.53.10.0/24
,
10.53.11.0/24
,
10.53.12.0/24
.
Still four networks, but 9 is not a multiple of 4, so they straddle two /22 blocks. No single route covers exactly these four. The tightest set is:
10.53.9.0/24, 10.53.10.0/23, 10.53.12.0/24
Paste either list into the route summariser and you will get the same answer.
The Rule
- The number of networks must be a power of two (2, 4, 8 and so on).
- They must be contiguous.
- The first must start on a multiple of that number in the interesting octet.
Practice
5 questions, new each time. Every answer is worked out by the same library as our subnet calculator, so if one looks wrong, check your working first and then tell us.