basic subnetting question

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Feb 3rd, 2009
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I am working toward my CCNA and studying on Cisco website for NAT translation. According to the scenario presented, is this correct?


S0 - 172.16.10.64/24

The available valid addressed which we can use (for NAT) are 172.16.10.1 - 172.16.10.62.


I would think the correct valid addresses for this network would be:

172.16.10.65 - 172.16.10.126


Why is this incorrect?

Thanks for any help.


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pstebner10 Tue, 02/03/2009 - 12:18
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Something looks wierd. Does this mean that the IP address of the serial (S0) interface is 172.16.10.64? If this is correct, with a 24-bit subnet mask, then .1-.254 are available (except for .64 since it is already in use)for NATing purposes. The range that they have specified would be valid for the network 172.16.10.0 /26, though then the S0 interface could not have the .64 address.

The range that you have specified (.65-.126) is 172.16.10.64 /26

They are actually both wrong. Can you post the link where you read this?


HTH,

Paul

Leo Laohoo Tue, 02/03/2009 - 13:34
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Let me break this down ...


S0 - 172.16.10.64 /24


Mask: 255.255.255.0 (/24)

Subnet: 172.16.10.0

Broadcast: 172.16.10.255


Ergo: Useable valid addresses are from


172.16.10.1 to 172.16.10.254 where 172.16.10.64 is a member thereof.


Does this help?

skypilott2 Tue, 02/03/2009 - 13:42
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If you go the link I posted above and view the example, now that I reconsider it, the following statement threw me off:


"The available valid addresses which we can use are in the range of 172.16.10.1 through 172.16.10.63"


With a /24 network, I don't think 172.16.10.64 can be a subnet ID --- as Paul said above, a .64 ID would require a /26 mask.

pstebner10 Tue, 02/03/2009 - 13:58
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I looked at the link - they are just defining a pool of available addresses for NAT (not overloading) to be .1 - .63. So, in effect, this example is correct. They could, however use the whole /24 network for NAT if they wanted to, minus the .64 (S0) address.


By the way, just defining a network of 172.16.10.64 does not necessarily mean that it has to be a /26 - this could be variably subnetted to whatever you need. /26 just means that you are using .65-.126 as available addresses, with .127 being your broadcast address. You could alternatively, for example, use 172.16.10.64 /27 or /28, /29, /30

for networks of 172.16.10.65 - 94, 172.16.10.65 - .78, etc.


HTH,

Paul

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