Summarisation Question
Aaron.Stones
Member Posts: 6 ■□□□□□□□□□
in CCNA & CCENT
What is the most efficient summarization of the following routes:
198.18.49.0
198.18.50.0
198.18.51.0
198.18.52.0
Is it 198.18.48.0/21 or 198.18.32.0/22.
My undestanding is that 198.18.48.0/21 is the correct answer as first five bits of third octet are common and last three are uncommon.
Correct me please if I am wrong.
Cherrs
Aaron
198.18.49.0
198.18.50.0
198.18.51.0
198.18.52.0
Is it 198.18.48.0/21 or 198.18.32.0/22.
My undestanding is that 198.18.48.0/21 is the correct answer as first five bits of third octet are common and last three are uncommon.
Correct me please if I am wrong.
Cherrs
Aaron
Comments
-
hectorjhrdz
Member Posts: 127
198.18.49.0
11111111.11111111.11110000.00000000
x.x.00010000.00000000 198.18.16.0
x.x.00100000.00000000 198.18.32.0
x.x.00110000.00000000 198.18.48.0
the suffix is /20 isn' it?
198.18.48.0/20 -
agustinchernitsky
Member Posts: 299
hectorjhrdz wrote:198.18.49.0
11111111.11111111.11110000.00000000
x.x.00010000.00000000 198.18.16.0
x.x.00100000.00000000 198.18.32.0
x.x.00110000.00000000 198.18.48.0
the suffix is /20 isn' it?
198.18.48.0/20
The approach is good but remember you have to compare the first IP given with the last one:
198.18.49.0 x.x.00110001.00000000
198.18.52.0 x.x.00110100.00000000
that 5th bit is the last common one: /21 is the summarization.
If you used a /20, you would be summarizing the first 4 IP blocks up to .51.255... but the .52.0 network would be left out... So you need an 8 IP block to summarize, then the /21 would do it (from .48.0 to .55.255) -
PCHoldmann
Member Posts: 450
most books/questions would probably say is was:
198.18.49.0/24
198.18.50.0/23
198.18.52.0/24
Because using a /21 covers 8 subnets (.48.0 - .55.255), which could cause problems for the other subnets.There's no place like ^$
Visit me at Route, Switch, Blog -
agustinchernitsky
Member Posts: 299
PCHoldmann wrote:Because using a /21 covers 8 subnets (.48.0 - .55.255), which could cause problems for the other subnets.
Correct! So, when subnetting care must be taken when asigning IP blocks. In this case, you must remember to reserve blocks .52.0 to .55.255 in the same segment / router as the rest of the IP blocks you are summarizing... if not... you will have a problem.
There is just this case in the 5th ed. book of Todd Lammle. basically he states: less is not good, more is good but remember to make a reservation.
