How to route a private network outside ISA firewall?

Posted on 2011-11-01
Medium Priority
Last Modified: 2012-05-12
Hi there, I'm fairly new to using firewalls in any sort of minorly complicated way, and am not sure how to configure it for this scenario.

I have two networks that share my internet connection on a small Cisco switch. My primary network uses ISA 2006 (for now) and let's say it has a public IP address of The second network uses a Cisco ASA, which has a public IP address of say

One of the requirements for the second network is that the primary network communicate with it through private IP address space.  The idea here is that my primary network can put stuff onto the secondary network without it going out through the internet, and then the customer using the secondary network can pull the stuff off using an encrypted tunnel.  The secondary network is configured to be the gateway ( for the private network, and there's another server in there that listens only on and for the sake of argument, FTP traffic (so port 21.)

From my primary network (which let's say uses 10.x.x.x), how can I route 192.168.x.x traffic through my ISA firewall?

I've tried using this route command to test it on the firewall:

route -f ADD MASK

If I try to FTP from my firewall server (e.g., "ftp"), the second machine rejects it because it's trying to access the FTP port on it's public IP.  What I need it to do is go through the network.

What might I be doing wrong?  Can I even do what I'm suggesting with ISA?

Thanks in advance for your help.
Question by:Gonthax
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Assisted Solution

by:Ernie Beek
Ernie Beek earned 300 total points
ID: 37061150
Well, there is a bit more to it then just adding a route. Private ranges can't be routed over the internet (public ranges).
If you want the two networks to interconnect, you want to set up a site to site vpn. Have a look at: http://www.isaserver.org/tutorials/Implementing-IPSEC-Site-to-Site-VPN-between-ISA-Server-2006-Beta-Cisco-PIX-501.html

If you only want to access a few services you can open the ports for those services (like ftp, ssh, http).
I think you in your case you want to look at the first option.
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Accepted Solution

ienaxxx earned 1000 total points
ID: 37061378
another solution would be to add a third Nic card to the isa firewall and then setup this network as an additional internal network or as a dmz net. Linking that ni to the second network would let both nets to communicate, accordingly to the rules you aetup in isa.

You can do the same with a third net interface of asa, or even connect two outside-configured nic to each other betw. Firewalls, but this is the hardest way.
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by:Fred Marshall
Fred Marshall earned 300 total points
ID: 37065213
I have two networks that share my internet connection on a small Cisco switch. My primary network uses ISA 2006 (for now) and let's say it has a public IP address of The second network uses a Cisco ASA, which has a public IP address of say

OK.  As I get it, these must all be on one site ... rather sitting side by side.  You've not told us the two private subnets so I'll have to fake it.  I'll assume /24 and /24 here:

There are a few approaches.  I'll start with what I think is conceptually the simplest one first:

Use a simple router like a Cisco RV042 to connect the two subnets.  Here I'm assuming that you don't need internet access to be "mixed" between subnets.
- Set up the LAN side to have an IP address in /24.
- Set the WAN wide up to have an IP address in
- Set the router in "Router" rather than "Gateway" mode.
- On each LAN gateway device, you add a route to the other subnet that points to the local IP address of the RV042.
This should be all it takes to allow communication between the two private-addressed subnets.

But, since you have those two Cisco boxes already, let'ss start with the one on
You may be able to set up a secondary private subnet on it that's on 192168.1.0/24 ... which will give at least one port an IP address on let's say that it's  And I believe that once set up you won't even have to add a route to the other subnet because it's connected.  But .... you may.
You plug this port into the other LAN.  I'm assuming that this box is the internet gateway for its own LAN/subnet.  

OK, so the other Cisco box is on already.  I'm also assuming that the other Cisco box is the internet gateway for the other LAN/subnet.
So, on that latter Cisco box you add a route for the other side that goes to the added LAN port on the other Cisco router: to

This way all the internet traffic goes through the "local" LAN gateway and all the traffic between LAN subnets goes through the Cisco gateways in both directions.

Packet leaves a computer on; goes to its gateway perhaps it's
From there, it's routed to
From there, its' routed to the LAN port, onto the wire and to the destination address on that LAN.

Responding packets will go to the "local" gateway, perhaps it's
From there the packets will be routed to it's other LAN port addressed and out onto the other LAN wire.

NOTE:  The first scheme with the RV042 is a little "unbalanced" in the sense that incoming packets get dumped out onto the LAN directly and returning packets have to go through the gateway.  Now, IF the gateway is using stateful packet inspection on the LAN side then the return packets won't have a state established and may be dropped.  So this feature has to be turned off.

The same thing applies to the second approach because the router dumps packets onto the wire on without hitting the gateway there.  So, return packets from, when they hit the "local" gateway won't have a state either.   So the feature has to be turned off on that one router.
But, packets originating at will go to and establish a state.  So there, returning packets shouldn't have a problem.

I guess one slightly different approach would be to route the packets from going to to instead of to  Then a state would be established in the router.  But, I'm not sure that will work as .99 will be in the routing table likely anyway. It may become equipment specific.

I mention all this because you have two different devices
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pwindell earned 400 total points
ID: 37070002
Well,...I'm going back to the original question.  It is the only way to make sense of this.

Undo everything you did and put it back the way it all originally was.  Everything you did was the wrong approach.

You have about 3 options.  I'll give what I think is the best and most solid method first.

Option 1
You should have a LAN Router between the two LAN Segments,...that's what LAN Routers are for.  It gets worse from there,...because each LAN segment uses its own independent path to the Internet (you the ISA, the other guys the ASA) there needs to be two LAN Routers positioned "back-to-back" with a single point-to-point link between them.  Each LAN uses their own LAN Router as their Default Gateway and the LAN Router uses their own Firewall for its Default Gateway.  It looks like this:

Option 2
It is possible to create a point-to-point LAN link between a 3rd interface on the Firewalls, but that can get really complicated if you are not an expert in dealing with the firewalls.  It would look like this:

 Drawing 2
Option 3
A third option is a Site-to-Site VPN between the two firewalls.  It would behave the same as a point to point LAN Link between the firewalls.   It can also get really complicated if you are not an expert with dealing with Firewalls and VPNs.  It would look like this:

 Drawing 3

Author Closing Comment

ID: 37095697
I want to thank you all for your contributions, and I decided to split the points up amongst you guys because they all seemed like valid solutions to my problem.  Ultimately I ended up going with ienaxx's (and basically pwindell's option #2 - btw pwindell, awesome network image layouts, what did you use? :))

What worked was setting up essentially a DMZ with local IP's, and that worked great.
LVL 29

Expert Comment

ID: 37095734
I used Miscrosoft's Visio 2007.
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Expert Comment

ID: 37098745
Glad you found a solution. :-)

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