Summarisation Question

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

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Comments

steve-o87

Yup that's right!

Aaron.Stones

Thanks.

hectorjhrdz

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

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)

hectorjhrdz

wtf!!!!!!!!!! i didn't remember that!!!
you're right so

thnks dude

PCHoldmann

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.

agustinchernitsky

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.