ASA 5505 & Multiple external IP

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Jul 24th, 2010
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I have a business Broadband, which has 5 IP addresses spread over 3 class C's supernetted for it to work each IP needs to be registered to a unique MAC address.
Is there any way I can get my ASA 5505 to have all 5 ip's on one interface, listening with 5 separate MAC's

Range - /22

IP1 MAC 0000:0000:00A0
IP2 MAC 0000:0000:00A1

IP3   MAC 0000:0000:00A2
IP4 MAC 0000:0000:00A3
IP5 MAC 0000:0000:00A4

All on eth0 set as the external interface.

Thanks in advance


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David White Sat, 07/24/2010 - 06:44
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Hi Frank,

Unfortunately, this is not possible.  All IPs that are owned by the ASA will have the same MAC.  The only exception to this is for the standby IP in a failover set.

A follow-up question though.  Why does each IP need to be associated with:

  - a different MAC

  - a specific MAC




frankbailey Sat, 07/24/2010 - 11:22
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The way that my broadband works is that I get 5 IP addresses that are assigned to the cable modem, and you have to have a different MAC for each one. It would be better if they gave you a range of IP's, but they say that can't be done. So my 5 IP's are dotted about 3 subnets. I have to have a router for each one, which is a pain.
(ISP is a UK Cable Company)



David White Sat, 07/24/2010 - 11:53
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Hi Frank,

Unfortunately, in that case there isn't anything the ASA can do for you to help you utilize all 5 IPs.



Nagaraja Thanthry Sat, 07/24/2010 - 12:08
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If you really need to have 5 different MAC addresses, then you could

probably use a L3 switch module on a router (4 port or 9 port) and put each

port on a separate VLAN. That should allow you to register separate MAC for

each public IP and you will be able to use the firewall feature set on the



Hope this helps.



frankbailey Sun, 07/25/2010 - 02:44
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The 5 Vlan method was the first I thought of, but wasn't sure about. I will give it a whirl and report back.



frankbailey Mon, 07/26/2010 - 00:10
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5 Ports in 5 Vlans doesn't work.

1st interface goes in OK, but once you get to the 2nd it fires an error pointing out that they are infact on the same subnet.

Back to Square One. But thanks for the help.



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