How do I configure a Cisco ASA 5510 with multiple subnets and multiple external IP addresses?

Posted on 2009-07-10
Medium Priority
Last Modified: 2012-05-07

Here is my problem. I have been a long time windows admin and have stumbled into cisco as a job requirement when a cisco asa 5510 was bought at my new job. I am starting from scratch and have read all over the internet and cisco's technical examples and configurations and have found nothing. What I need is each physical interface to be its own subnet: i.e.

etho0/0 would be the outside interface but with multiple external static IP addresses brought in on fiber handed of via ethernet
etho0/1 would be subnet and all traffic allowed out from the users and specified traffic nat'd through to a specific server/servers from etho0/0 to one maybe two external IP's for email, RDP FTP and web VPN and a few other services.

etho0/2 would be Same thing as above

and etho0/3 would be but this one would have 3 external ip's from the etho0/0

The final catch is none of the subnets can talk to each other.

Can this be done? If so how would I go about mapping the external IP address to each subnet and keep them from seeing each other.

Sorry if this is an easy stupid question.

Question by:cisconoob
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LVL 33

Expert Comment

ID: 24826119

Welcome to the world of cisco.  No worries we all started somewhere....  

The external interface would be set with the public internet ip that will service the range of IPs from the ISP.    

The multiple internal interfaces or internal VLANS will each support 1 of the 3 subnets you described.     You can do either physical separation of the subnets or a VLAN interface for each off the physical inside interface trunked into a switch.  

With the ASA each innterface is given a security level so that lower security numbered interfaces will not communicate with higher numbered interfaces unless ip are mapped and Access lists created.  

Access lists will control the flow of the packets from interfaces.  Access lists are written and then applied to the interface in a certain direction in or out from the perspective of the ASA.    You can have only 1 ACL in 1 direction on any interface at 1 time.  

I suppose the best way is to have you go to the howtos that cisco has available.  

To get started, you should have a look at these:

Here are a bunch of examples covering that should help as well:


Author Comment

ID: 24826549

Thanks for the primer. Thats a great starting point but I think I may need to be a little more specific
and my etho0/0 would be a xxx.xxx.xxx.114 ip address. With this and the code below setup correctly with an acl to allow the correct traffic in would that work? and will this allow all users on these subnets OUT with out any issues no matter what they are doing as well as not see each other?

interface Ethernet0/0
 nameif outside
 security-level 0
 ip address xxx.xxx.xxx.114 
interface Ethernet0/1
 nameif inside1
 security-level 100
 ip address 
interface Ethernet0/2
 nameif inside2
 security-level 100
 ip address 
interface Ethernet0/3
 nameif inside3
 security-level 100
 ip address 
interface Management0/0
 nameif management
 security-level 100
 ip address 
passwd 6kRwdQPEOenirCCd encrypted
ftp mode passive
dns server-group DefaultDNS
 domain-name test.com
object-group service Email tcp
 port-object eq smtp
 port-object eq www
access-list outside_access_in extended permit icmp any any 
access-list outside_access_in extended permit tcp object-group Postini host xxx.xxx.xxx.116 object-group Email 
access-list outside_access_in extended permit tcp any host xxx.xxx.xxx.117 eq pptp 
access-list outside_access_in extended permit tcp any host xxx.xxx.xxx.115 eq domain 
access-list outside_access_in extended permit tcp any host xxx.xxx.xxx.118 eq telnet 
access-list outside_access_in extended permit gre any any 
access-list outside_access_in extended permit tcp any host xxx.xxx.xxx.120 object-group Email 
pager lines 24
logging asdm informational
mtu outside 1500
mtu inside 1500
mtu management 1500
icmp unreachable rate-limit 1 burst-size 1
icmp permit any outside
asdm image disk0:/asdm-602.bin
no asdm history enable
arp timeout 14400
global (outside) 1 interface
nat (inside) 1
static (inside1,outside) xxx.xxx.xxx.120 netmask 
static (inside1,outside) xxx.xxx.xxx.115 netmask 
static (inside2,outside) xxx.xxx.xxx.117 netmask 
static (inside3,outside) xxx.xxx.xxx.118 netmask 
static (inside3,outside) xxx.xxx.xxx.116 netmask 
access-group outside_access_in in interface outside
route outside xxx.xxx.xxx.113 1

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LVL 33

Expert Comment

ID: 24826930
In order for users from all interfaces to get out, you would need to add them to the NAT command like so:
nat (inside1) 1
nat (inside2) 1  

That means the nat will also take any traffic from those interfaces and use the global 1 to translate them.  

The other statics and ACLs look good.  
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Author Comment

ID: 24826940

Thanks, I'll report back in a bit to let you know once I put it live (fingers crossed)
LVL 33

Expert Comment

ID: 24839514

Author Comment

ID: 24839951
I've was called out of town this weekend to another site..I am trying the implementation tonight....I'll post back soon

Author Comment

ID: 24843049
Ok so the situation changed a bit....the client wants things done a bit different now. on one of the interfaces they want a complete IP passthrough well let me see if i can explain this better...they want there outside ip xx.xx.xx.111 passed off inside so in essence it would hit the firewall and then be passed off to them internally and assigned to there linksys that they are upgrading to a cisco pix. so it will go ISP > asa 5510 > linksys > xx.xx.xx.111 so on the interface of the linsys they want the ip to be xx.xx.xx.124 can this even be done?
LVL 33

Expert Comment

ID: 24843139
The asa can be placed into transparent mode which would give you that behavior.   Ill have to lookup a link at cisco for a howto

Author Comment

ID: 24843186

Well I can't place it in transparent mode because we are still splitting of two subnets. Thats the catchy party. One subnet I can split off the asa directly and manage the other company wants to connect there equipment to the cisco asa with the passthrough as decribed above....uggggg
LVL 33

Expert Comment

ID: 24843238
I don't think that will work.   Another expert may want to chime in here, but the asa can't have 2 directly connected items with the same ip.  With distant networks sharing subnets, you can use the nating feature on each end, but that's a different scenario.    I think that's a no go

Author Comment

ID: 24843288
Thats what i thought but let me try and be more specific....NO sharing between each other I want each network to have its own external ip xx.xx.xx.76 for my network xx.xx.xx.77 for our neighbor business they want there own router/vpn end point and want a straight pass through to the linksys


pretty much the diagram in the link but the issue is on the node in the diagram they want there own external IP .76 to be passedthrough to them directly...hope this makes sense
LVL 33

Accepted Solution

MikeKane earned 1500 total points
ID: 24851585
I suppose you could double STATIC NAT the address Once coming into the Firewall and out the inside 1 port to a non-public range,  then another static at a 2nd device to turn it back to the public IP.        

Honestly, I don't know what complications that might bring with it as I've never tried that before.  

Author Comment

ID: 24852163
Thanks Mike this is obviously an over complication and just need to go back to the drawing board....I will award you the points and thanks for the help...

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