Cisco PIX 501-50 need External IP Addresses to be used Internal

Posted on 2004-10-19
Last Modified: 2012-06-21
The company I work for recently leased rack space from a local datacenter in order to co-locate a few of our client’s servers there.

After doing a bit of research we decided to purchase a Cisco PIX 501-50 to go in front of the 3 servers (all running Linux) we are going to co-locate. One problem: I am not Cisco trained nor do I care to be (Although I respect those that are.).

The servers behind my PIX are going to be used mainly for web applications that our company develops. We are also going to also place a server of our own in the rack to sell web space to end users.

I know that I could assign internal 192.168.x.x addresses to our machines behind the PIX. This would be the easiest scenario, and believe me I would love to do just that. The problem is that all three of the servers behind our PIX need external (real internet) IPs assigned to them. We have about 50 Internet IPs that the data center has assigned to our rack.

Here is my diagram:

Internet IP Range ----->  PIX -----> Server 1 (
                                                                              Server 2 (
                                                                              Server 3 (

I still need the PIX to do firewall duties such as forwarding / blocking ports and things of that nature.

Anyone know how to make this happen? I know the Netscreen can do this (as I have a managed server on RackSpace that has an external IP behind a Netscreen) but I don’t know how (or even if it is possible) to make the Cisco PIX 501-50 do this.

Another side note: The PIX is sitting here in front of me in the box still. We deploy next week, so I can’t do any trial and error troubleshooting.
Question by:chrisswain
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Expert Comment

ID: 12350945
I am not sure what you mean by you need to assign external IP addresses to them (and why), but if you have 3 external IP and you want to point them to the appropriate servers that you want, you can always use NAT to do so.

       Server 1 -
       Server 2 -
       Server 3 -

in the PIX firewall, Configuration->Translation Rules, you can set up new static route rules to translate the external IP address to the servers -> -> ->

and so on for the rest.
Hope this helps..

Author Comment

ID: 12351217
Yeah, I understand that I can assign 192.168.x.x addresses on the LAN side, and then setup static routes to the internal IPs.

I need to assign external (live Internet IPs) to the internal machines. We are running software that requires an external ip addresses to be assigned to the adapter interface. Like is on Server1's eth0

Could I turn NAT off and somehow translate 123.123.123.x to 123.123.123.x? I really wouldn't need NAT as I am using real internet IPs would I?

Expert Comment

ID: 12351501
I tried to see if setting the same IP in both inside and outside interfaces will work but it was a little bit weird, you can give it a try if you have time and are able to do that (I don't want to mess up my network here or I will be in trouble :D)
On the other hand, ever thought of having one more router/PIX which is doing the reverse translation? with 192.168.x.x on the outside interface with the 123.123.123.x in the inside? This would probably work.. but I know.. one more router..
Hope this helps..
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Expert Comment

ID: 12352206
The command to make a static NAT translation in the pix is:

static (inside,outside) 123.123.123.x (outside IP address) 192.168.0.x (inside address)

You would have to put one in for each address to be translated (3 in your case).  You would also have to open up the firewall to allow web traffic in to those IP's which is a whole other topic.  

Expert Comment

ID: 12354800
Did they give you a network range? If so, you can subnet it down and give valid public addresses inside.

Original block by ISP, usable
Subnet to and

Then use on the outside and on the inside. You lose some IP's by subnetting but you can then use public IP's on the inside.

You'll have to put in NAT exceptions as the PIX will want to NAT by default.

Accepted Solution

ewong_111 earned 500 total points
ID: 12367444

Here's a creative way of dealing with this problem but I am not sure if this works.. but why not give it a try if it saves you any addition expenses.. :)
try to assign 2 ips for the NIC, one internal and one external (btw, I only tried this on Windows machine, not sure if it works for Linux)
let me know if this actually works..
best regards..

Author Comment

ID: 12930956
Sorry about not getting back to this question sooner (work has been keeping me very busy).

I solved the problem by giving my eth0 an internal 192.168 IP and setting up device aliases for my external IPs in linux. So I have an eth0 eth0:1 eth0:2 and so on. Here is a sample of my /etc/network/interfaces (keep in mind that I had to compile network alias support into the kernel)

Public IP's have been changed. :)

auto lo eth0 eth0:0 eth0:1 eth0:2
iface lo inet loopback

iface eth0 inet static

iface eth0:0 inet static

iface eth0:1 inet static

iface eth0:2 inet static

You can check out this URL for more information: 

I think that ewong_11 deserves the points as he suggested giving the interface multiple IPs. I just went and found out how to do it in Linux. Thanks!

-Chris Swain

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