summarization

First, when summarizing, the mask boundary is moved left, not right.

Of the four network in Phoenix, start with the lowest (10.4.0.0/24).

If the mask is moved 1 bit to the left (10.4.0.0/23), that summary route will include the 10.4.0.0/24 and the 10.4.1.0/24 networks. Or put another way, we now have a "summary field" (as opposed to a subnet field) of 1 bit. A 1-bit summary field includes two networks.

Moving the boundary one more bit (10.4.0.0/22), will create a summary route that includes the 10.4.0.0/24, 10.4.1.0/24, 10.4.2.0/24 and 10.4.3.0/24 networks. Or now we have a two-bit summary field which will allow 4 networks to be summarized.

Thanks. Let me go thorough your notes.

hi Akinsd,

i understand the notes. I still have few things need to be cleared. How would I able to do the following summarization question.

http://www.9tut.com/ccna-subnetting-questions-3

question 9 , i did apply the same method but not able to understand why the answer is B and D. Please explain.

You would need to look at the existing networks.

Core-Marketing  172.16.3.4/30  (2 hosts: 172.16.3.5 - 172.16.3.6)
Core-Sales  172.16.3.8/30  (2 hosts: 172.16.3.9 - 172.16.3.10)
Core-Admin  172.16.3.12/30  (2 hosts: 172.16.3.13 - 172.16.3.14)
Marketing LAN: 172.16.3.32/27 (30 hosts: (172.16.3.33 - 172.16.3.62)
Sales LAN: 172.16.3.64/27 (30 hosts: (172.16.3.65 - 172.16.3.94)
Admin LAN: 172.16.3.96/27 (30 hosts: (172.16.3.97 - 172.16.3.126)

So for Network A, a mask of /25 is needed to have host field large enough to accommodate the 120 hosts on the LAN.

For the link, a /30 mask is used.  Answer D works and the addresses 172.16.3.1 - 172.16.3.2 would be assigned to the router interfaces.

Answer E would have addresses which have already been assigned to Marketing LAN.

Answer F would have addresses which have already been assigned to Admin LAN.

sorry i mean to say question 9. Please look at the question 9 about summarization.

i am not quite sure how reverse route summarization works.

I think you're overthinking that one. :-)

Look at it like you would a regular subnetting question.

What are the valid addresses for the 172.31.80.0/20 network?

172.31.80.1 - 172.31.95.254

Any addresses in that range would be your answers.

question is /20 but how come they are using /22 in the answer?

this is where i am having confusion.

The /20 is a summary route which contains a number of smaller networks. Just like 172.16.0.0/16 would be a summary for the 172.16.0.0/24, 172.16.1.0/24, 172.16.2.0/24, 172.16.3.0/24, etc.

Thanks for the points and feedback.
Let me know if you still need help or clarification