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Multiple dynamic IPs from ISP?

Unanswered Question
Apr 7th, 2003
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Hi! I have been asking this couple of times now but no solutions for my problem has come up yet.

Is it possible to acquire all the available IP numbers from ISP to the Cisco 2600 router with ADSL WIC ? My ISP is providing 5 dynamic public IP addresses for end users via DHCP. I'd like to have all of them on my router so that I could do different kinds of routing for different IPs and I could assign some of them for specific purposes only!

CAN this be done? If so, how? I'm running IP/FW/IDS PLUS IPSEC 3DES software v12.2.11T6.

Currently I have been trying to do it something like this way:

bridge irb




interface ATM0/0

no ip address

no atm ilmi-keepalive

pvc SoneraADSL 0/33

encapsulation aal5snap


dsl operating-mode ansi-dmt


interface ATM0/0.1 point-to-point

bridge-group 1


interface ATM0/0.2 point-to-point

bridge-group 2


interface ATM0/0.3 point-to-point

bridge-group 3


interface ATM0/0.4 point-to-point

bridge-group 4


interface ATM0/0.5 point-to-point

bridge-group 5


interface BVI1

mac-address 0000.0cbc.b751

ip address dhcp


interface BVI2

mac-address 0000.0cbc.b752

ip address dhcp


interface BVI3

mac-address 0000.0cbc.b753

ip address dhcp


interface BVI4

mac-address 0000.0cbc.b754

ip address dhcp


interface BVI5

mac-address 0000.0cbc.b755

ip address dhcp


bridge 1 protocol ieee

bridge 1 route ip


bridge 2 protocol ieee

bridge 2 route ip


bridge 3 protocol ieee

bridge 3 route ip


bridge 4 protocol ieee

bridge 4 route ip


bridge 5 protocol ieee

bridge 5 route ip

If I configure only BVI1 with ATM0/0 being in bridge-group 1, it works for one dynamic IP. I believe the PVC is not inherited to subinterfaces? I cannot assign same PVC to multiple subinterfaces (ATM0/0.x)... It also works if ATM0/0.1 for example is having the PVC 0/33 and being member of bridge-group 1, but then the ATM0/0 must not have the PVC.

ANY help appreciated!! I'd like to know at least if it can be done or not so that I know whether to waste more time or not :)

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tepatel Mon, 04/07/2003 - 19:13
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  • Cisco Employee,

No..you can't get 5 ip address like that. You can configure the router like that to inherit the pvc.

Typically isp allocates a subnet of ip address to users like you have 5 ip address. But those will not be allocated to interfaces like that.

It will only allocate one ip address and route rest 4 ip address to that one ip address allocated to interface. So isp will install a route to the subnet allocated to you with gateway of the ip address given to that interface. That way packets destined for those 4 ip address will be forwarded to that one ip address and your router will manage the rest.

So go ahead and use those rest 4 os the ip addresses the way you need. Just configure the router for one pvc and interface. ISP will manage the routing for rest of the ip's.

kimtuomi Mon, 04/07/2003 - 23:54
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I think you have replied me previously with something like this, if I recall correctly. Anyway, the problem here is that I do not have any static IP addresses or subnet or whatsoever. The ISP is not willing to give static IP addresses for these "home" connections. They provide each customer with 5 dynamic addresses. The subnet mask for that comes from DHCP is something like or so (I'm not near my setup on time of writing). The default gateway for every client is same (if I remember, Lease time for dynamic addresses is 3 or 4 hours.

I can, for example, do this:

Create a bridge group of ATM interface (with the PVC defined) and FastEthernet interface. I create a corresponding BVI and can get one of the IP addresses to that (ip address dhcp). Then, behind that FastEthernet I put a switch or hub and connect four client PCs to that. Each of these PCs will get ip address. Then all my IPs would be in use. If I connect fifth PC, it does not get an address unless I release one of the previous ones.

This gave me the idea that I'd like to have all of the addresses assigned to router and then there, in single point, I could filter the traffic and put it to go where I want. I'd like to run my internal network completely with private addresses (192.168.1.x/27). I need at least one of the IP addresses with NAT to be used only for IPSEC VPN connections for a VPN security gateway device behind my router.

kimtuomi Tue, 04/08/2003 - 00:26
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I just noticed that in this forum, I am able to see all my own threads/conversations in a neat list and I do not need to search for my post in the complete huge list :) Nice.

So, I checked the previous answers I have had. Here is a discussion where you replied to me:


Here you indicate the use of NAT pool but that, too, requires that I can define the static IP addresses. Or can I create this NAT pool thing to be able to use dynamically assigned ones?

I am really hoping to find some kind of solution for this; there is bigger plans for the usage of all these IPs. Most important thing being the VPN connections.

Also, someone else told me that I should be able to create multiple subinterfaces for the ATM0/0. Then attach these interfaces to different bridge-groups.

If I define only ONE subinterface for ATM0/0 and that subint is a member of bridge-group 1, I still cannot get the IP address for BVI 1 with "ip address dhcp". Why is that? IOS reports something like "no PVCs available for bridging" or so... I mean, if subint should be working with main interface's PVC, it should be working like this (getting the IP, passing traffic). If I remove PVC definition from ATM0/0 and create that under the subint which is part of bridge-group, it works this way. BUT, I cannot define the same PVC 0/33 for all the subinterfaces (it moves to PVC configuration mode of the existing one automatically).

tepatel Tue, 04/08/2003 - 07:58
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  • Cisco Employee,

You need different pvc's for different sub-interfaces. So one pvc for all the other sub-interface will not work at all.

Now to resolve the issue of ip addresses, you need 5 fixed ip address. So that you can assign them to some servers


you can configure some static NAT on. I don't see any other way to fix this issue.

kimtuomi Tue, 04/08/2003 - 12:43
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If I get the static IP addresses, where do I configure them? As you said, directly for different network devices like server and VPN gateway?

Or then, with static IPs, I could create the NAT pool and then do some static NAT directly between public IP <-> private IP ?

I don't quite get it why it could not be possible to acquire multiple dynamic IPs for IOS. Maybe in future IOS version or is it just impossibility?

Thanks for the knowledge so far!


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