Subnets. Private or not?

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Mar 7th, 2008
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Hey guys, just to double check, my understanding of - is that is one of the 3 private networks that the RFCs provide us so we can use them internally (non-routable IPs).

This means that I can pick any from 172.16.x.x all the way to

What happens if in my internal LAN I use 172.15.28.x?

what could be the problems/?

Also, if the config has statements like:

ip address DMZ

static (DMZ,outside) netmask 0 0

What could be the problems here?

please help

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thiru.vel10 Fri, 03/07/2008 - 19:03
User Badges: this is not a private IP address. IF you using this IP range in your LAN it will create routing loops in your network.

Jon Marshall Sat, 03/08/2008 - 01:40
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You can use any range you want on your internal LAN including routable internet address ranges.

But if you do use a publically routable address range that has not been assigned to you there are 2 things you need to be aware of

1) By using 172.15.28.x on your LAN it means you will have problems accessing any 172.15.28.x servers on the Internet. So if the company who has been assigned the 172.15.28.x has a web server you want to access this could be problematic.

2) You must make sure that none of the 172.15.28.x addresses you have used "leak" out onto the Internet. So you would be fine with your DMZ setup because you are presenting your internal address as



insccisco Sun, 03/09/2008 - 17:34
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From the design perspective, will this be "bad" design?

What about the route looping like it was mentioned?

Edison Ortiz Sun, 03/09/2008 - 17:41
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It's a bad design if you don't own the 'public' IP address.

Jon did a great job explaining what would happen if another company owns the public IP and you decide to use it internally so I won't repeat his statement.

No route looping on such design, just the inability to reach internet sites as your router prefer those routes internally.




insccisco Sun, 03/09/2008 - 17:45
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Thank you guys for making this very clear.

I needed to be 300% sure as this is an inhereted L3 device and I keep finding strange things in the config.



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