basic subnetting question

Unanswered Question
Feb 3rd, 2009
User Badges:

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

S0 -

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

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

Why is this incorrect?

Thanks for any help.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
pstebner10 Tue, 02/03/2009 - 12:18
User Badges:

Something looks wierd. Does this mean that the IP address of the serial (S0) interface is 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 /26, though then the S0 interface could not have the .64 address.

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

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



Leo Laohoo Tue, 02/03/2009 - 13:34
User Badges:
  • Super Gold, 25000 points or more
  • Hall of Fame,

    The Hall of Fame designation is a lifetime achievement award based on significant overall achievements in the community. 

  • Cisco Designated VIP,

    2017 LAN, Wireless

Let me break this down ...

S0 - /24

Mask: (/24)



Ergo: Useable valid addresses are from to where is a member thereof.

Does this help?

skypilott2 Tue, 02/03/2009 - 13:42
User Badges:

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 through"

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

pstebner10 Tue, 02/03/2009 - 13:58
User Badges:

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 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 /27 or /28, /29, /30

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




This Discussion