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NAT to an ip address that isn't directly connected to a PIX 515


Can an ip address be NAT'ed to an ip address on a PIX 515 which isn't an ip address of a network directly connected to an interface on the PIX?

Specifically, can a host with an ip address of which is connected to a network whose PIX firewall interface is be NAT'ed to an ip address of which is not a ip address of an interface that is directly connected to that same PIX 515?

I've attached a PDF depicting the network topology and describing the above.

My first response to this question is that it can't be configured to do this, but I'd appreciate either a confirmation or correction to this.


2 Replies 2

Jouni Forss


To my understanding this should be possible. That is, if I understood the setup correctly.

I'll give an example using your IP addresses/networks (no mask provided for the NAT address so I just used some mask)

I'll assume the router in front of the PIX is Cisco also and the port facing the PIX is FastEthernet0/1 and the PIX interface is FastEthernet0/0 (cant remember how names were on PIX) (Also depends if you running software 6.x or 7.x)


interface FastEthernet0/1

description Link to PIX

ip add

ip add secondary


interface FastEthernet0/0

description Link to Router

nameif outside

security-level 0

ip add

route outside

global (outside) 10 interface

nat (inside) 10

static (inside,outside) netmask

To my understanding the above should be possible, but the router in front of the PIX will need to have the NAT network under its interface too.

- Jouni

Yes this will work, we use it on one of our firewalls.

Your router in front of the firewall need to (as Jouni states) needs to have an ip address in the range and in the same layer2 segment/VLAN as the PIX's outside interface. This is because as soon as the PIX has static NAT configured (even on subnets that have NOT been configured on its interface), it will perform proxy arp for these IP addresses. 

So as soon as you configure NAT on your PIX outside interface using anything inn the range, your PIX will perform proxy arp on those IP adresses.

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