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Cisco PIX - all interfaces on same subnet - possible?

Posted on 2002-06-11
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Last Modified: 2013-11-16
hi guys,

Would like to ask you if Cisco PIX can do a "drop-in configuration mode" i.e. allows you to drop a firewall(Physically between the Internet router and the internal LAN) into an existing network without subnetting or changing the subnet of the LAN hosts.

Basically which means the network interfaces are all from the same IP / network segment.

Also, would also like to confirm the routing given that the above is possible:
(1) Outgoing: the default gw for the PIX going to be the router?
(2) Outgoing: the hosts default gw going to be the inside interface of the PIX?
(3) Incoming: how does it work from the router point-of-view? Any routing required here?

Pls advice..! Thxs








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Question by:Haho
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lrmoore earned 400 total points
ID: 7070015
No can do with PIX. Each interface must be a different subnet.
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by:Chriskohn
ID: 7071572
Hello Haho:
 Agree with Irmoore that you would need at least two subnet in order to define one security level being different than another.
 In answer to your other three questions:
1)Yes. You are correct in your thinking here.
2)Yes again you are correct.
3)Your router needs a default route to the Internet if that is way your going from here, or if still in an Intranet your routing may be static or dynamic depending on what you are trying to accomplish.
 Just as a suggestion to get around your dilemma of not having a second subnet, Perhaps you could try to use VLSM (variable length subnet masking) if your routing protocol supports it and if you aren't already. Depending on your IP addressing scheme at this time perhaps you could possibly provide a 2 IP address subnet just for your outside PIX interface and your router's Ethernet connected to it. Good luck, hope this helps Chriskohn
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by:Haho
ID: 7071668
I found a posting that says that it can be on the same subnet.

Pls refer to:
http://www.geocrawler.com/archives/3/90/1997/5/0/373659/

> >We were informed that Cisco PIX 4.0 can perform the following :
> >1) PIX is transparent, the 2 network interfaces on PIX can be configured
> >   using IP addresses from the same network segment.
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by:lrmoore
ID: 7071779
The date of that posting was over 5 years ago and also states:
> This really would not make sense...  if the internal and external
> networks are on the same segment, then there`s no point in having
> the firewall.

It still doesn't make sense.

PIX 4.0 has come a long way, baby. The whole concept of the PIX is that you have one network at one security level and other networks at either higher or lower security levels. You cannot have two interfaces at two different security levels on the same subnet.

My opionion has not changed. No can do. Sorry if you don't like it.
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by:Haho
ID: 7072384
ok, thanks!
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