From: Shine Joseph (shinepjoseph@iprimus.com.au)
Date: Fri Mar 14 2008 - 17:13:17 ARST
Matt,
Also, in your calculation of wild card mask, there is a mistake.
10.12.13.11
10.12.14.201
00001101 00001011
00001110 11001001
------------------
00001101 00001001 = AND (Network portion) = 13.9
00000011 11001010 = XOR (Mask Portion) = 2.204
---->>>>>> This must be
0000 0011 1100 0010 - 3.194
I can understand your confusion about the mask potion of summary addresses
and the wild card (Inverse) masks used in ACLs.
Have actually tried to put a summary address such as this one in a router
10.12.13.9 0.0.2.204? It doesn't accept this one. What is actually a summary
subnet mask? It just tells the router that I don't care about the last so
many bits of my address space. Instead you are trying to say I don't care
about the some bits from here and some bits there. The idea of subnet mask
is flawed then. So, the subnet mask portion must be contiguous.
For the summary address, your example works out to be
10.12.13.9 255.255.252.0
Now the wildcard mask for the ACLs is telling the router to match an address
range. This address range 10.12.13.9 0.0.3.194 (there is a mistake in your
calculation) is actually telling, I don't care about the first and second
bit of the address range of the third octect and don't care about bit
potions 8, 7, 4 and 2 from the forth octet. It could be a 0 or 1. So it
covers actually 4 addresses from the third octet and 16 addresses from forth
octet.
So, the possible addresses are
10.12.0.9
10.12.0.10
10.12.0.73
etc.
10.12.1.9
10.12.1.10
...
10.12.2.9
10.12.2.10
...
10.12.3.9
10.12.3.10
10.12.3.73
10.12.3.75
10.12.3.137
10.12.3.139
10.12.3.201
10.12.3.205
Without giving the specific addresses it's hard to say why sometimes there
are two lines for the summarisation.
HTH,
Shine
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Matt
Bentley
Sent: Saturday, 15 March 2008 1:23 AM
To: Cisco certification
Subject: VERY confused on summarization
Hi Team:
I know my knowledge here is a little lacking, but I really need some help.
I just cannot seem
to get a handle of summarizing routes. I have a few questions I would be
VERY grateful to have
answered.
When I am told to summarize routes overlapping as little address space as
possible, sometimes I see answers where there are two lines. Why are there
two lines. How do I know I shouldn't just do one line? Is there something
about major network boundaries that I don't know?
Also, I have read lots of documents on how to do binary xor/and, but it just
doesn't make sense when I do it.
For example:
10.12.13.11
10.12.14.201
I know the first two octets are the same, so I don't have to worry about
them. So, I take the binary of the third and fourth octets.
00001101 00001011
00001110 11001001
------------------
00001101 00001001 = AND (Network portion) = 13.9
00000011 11001010 = XOR (Mask Portion) = 2.204
So, is the summary 10.12.13.9 0.0.2.204 ????
What am I doing wrong.
How do I know that the mask should be zeros instead of 255's to start out
with? Something to do with access-lists?
Could someone show me an example and the theory behind knowing when you need
two statements
to summarize a certain address range as opposed to one? Is it a hard and
fast rule, or just a best practice sort of thing?
Thanks very much in advance.
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