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Public IPs, how to use less on an interface

Unanswered Question
Jul 25th, 2012
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I have a 2621 that I am configuring on the internet. My ISP gives me a static DHCP assigned address and then two more static addresses that are not part of the same block. (e.g. is static via dhcp and then they give me

I have fa0/0 getting ia dhcp. I have on a loopback interface for PAT/NAT as I have the main one on fa0/0 doing vpn to a remote ASA.

The problem is that I have yet another device that needs a public IP, mainly I want to hook that device up to fa0/2 (this box has three fa interfaces). How do I setup fa0/2 if I want to give the device on it a real live public IP address? I have done this before, but it must have been 10 years back on an even older CISCO and I can not remember how I did it.

Any help would be appreciated.


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nkarthikeyan Wed, 07/25/2012 - 11:38
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You have got one public IP via DHCP on your outside interface. /30 will have 2 public IP's which you can use for different purpose. I don know why you are using it for loopback. You can use the outside Ip itself for both inetrnet/vpn using PAT. Other 2 IP's you can have as the spare for other connections. let say on fa0/2 you connect some other infra with RFC private range and get that NAT/PAT using the spare 2 IP addresses.

Please do rate if this information helps you.



rminvielle Wed, 07/25/2012 - 15:16
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I have tried many times to get PAT/VPN working out of the same interface with a single public IP... It was mentioned to my by a CISCO employee that it would not work, and to assign a public to a loopback and use that for the PAT or VPN. That worked... so now I have one free public IP address.

I tried to statically NAT that single public to a private and assign that private to the internal router that is to get on

the net... that did not work.

So, I have one free public IP and I need to plugin a small router behind the 2621 and get it on the net and reach it

through a public IP address on the outside of the 2621... I know I have done this before, but I can not recall how.


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