? - Adding a new IP Block to My Cisco Series 1800

Unanswered Question
May 10th, 2007

We were assigned a block of 5 IP addresses to use from our Internet Service Provider and recently had to request another block of 5 IP's. We are using a Cisco Series 1800 router and have access to it remotely via a browser/gui.

Unfortunately when this was originally ordered the person didnt request remote management so we have to do this ourselves.

Lets say we were originally given a block of We were recently given a new block of Now as a Cisco noob I dont understand how this new block would be added to the interface settings on the router. I'd understand if the new block was a contiguous set of IP's off the original block but how do we add this new block to the Series 1800 interface?

The settings in the GUI do not seem to allow the ability to add a block of IP's like this. Does this have to be done command line? If so how do we do that? We have very limited Cisco experience here but we have enough knowledge to get the job done if someone can explain where and how. If there's anything I need to provide here that will better explain the situation please let me know. Thank you in advance for any help you experts can give! :)

I have this problem too.
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Jon Marshall Thu, 05/10/2007 - 04:11


What are you using the new IP addresses for. Generally speaking you would use public IP addresses for NAT purposes.

If this is what you want to do you do not need to add one of the new addresses to your interface. All you need to make sure is that the ISP is routing that new subnet to your router.

Then you can just use normal NAT commands on our router.

Does this make sense ?



LongUserName9000 Thu, 05/10/2007 - 04:18

Wow what a fast response :) Thank you.

We have web servers and other things that need to be accessable from the internet so we need to assign publicly available IP addresses. Normally in a situation like this...at least with our other T1 and ISP the router gets managed and adjusted by the ISP. In this case we are supposed to do it ourselves. If I understand what your telling me then we dont really need to do anything to the router at all, correct? If its a NAT setting then how would we do that?

Thank you so much for the fast response!

Jon Marshall Thu, 05/10/2007 - 05:21


You need to decide what device you are going to the NAT on. if you have firewall behind your router you can do the NAT for the web servers there. As long as the router sends the traffic for these public IP addresses to the firewall it will work.

If you are doing the NAT on the router then you will need to configure the NAT statements on the router.

Where are the web servers in relation to your router ?


LongUserName9000 Thu, 05/10/2007 - 05:36

Its setup like this:


Jon Marshall Thu, 05/10/2007 - 11:00


Just to clarify. Do you mean


If so you can do it on either the router or the firewalls, generally it's done on firewalls.

What you need to do is to create NAT entries on the firewalls for these web servers. Then you need to make sure there is a route on your 1800 for the new subnet range pointing to the external interface of the firewall.

This should be all you have to do. Plus rules on the firewalls for traffic obviously.

HTH, let me know if i have misunderstood setup



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