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Community Member

Cisco 1601-R w/ 2 IP blocks?

I have a Cisco 1601-R router. I have a primary static IP block from my

ISP configured in the router. I am currently using all available IP's

in the primary block. My ISP recently assigned me a second set of IP's

for use. Is it possible to configure my router to accept IP addresses

from 2 different IP blocks, or do I need a second router for this?

Thanks. P.S. I'm a cisco newbie.


Re: Cisco 1601-R w/ 2 IP blocks?

Not sure if I understood correctly.

But you can peer with your ISP using different IP addresses with no problems.

check an example:


Community Member

Re: Cisco 1601-R w/ 2 IP blocks?

I saw this answer on a messageboard somewhere:

hostname(conf-if)# ip address secondary

Supposedly, adding this to my config will allow me to start using an additional block of IP's from another subnet mask. Any truth to this?


Re: Cisco 1601-R w/ 2 IP blocks?

Yes that is correct. You can use secondary addresses to have multiple IP pools assigned to an interface. But before you do that I have a few questions ... Are you using public addresses directly on your hosts or do you have some form of NAT enabled? If you are using the public IPs directly on the hosts then it might be a good idea to think about changing that to private addressing and doing NAT on your router. This will alleviate the public IP requirement to a great extent and you might be ok with your original pool of addresses.

Community Member

Re: Cisco 1601-R w/ 2 IP blocks?

I have no NAT enabled, and I don't really know much about it.


Re: Cisco 1601-R w/ 2 IP blocks?

If you want a short term solution then the secondary address approach will work like I mentioned in my previous post. In the meantime I would advise that you start reading up on NAT and how you can benefit from it in the long run. What it will allow you to do is to have RFC 1918 (private addresses) addressing scheme inside your network. This basically means that you do not have to ask for a new block of addresses each time your public ip pool gets exhausted. Then you configure your router to NAT the inside RFC 1918 addresses to your public pool of address (or multiple pools if available). In this manner you can have hundreds of hosts inside your network and still use a limited public IP Pool. I will not go into much further detail as to how NAT works as you can find many resources on the internet that can point you in the right direction. You can use the following links as an initial starting point and work from there:

Re: Cisco 1601-R w/ 2 IP blocks?

here is the few basic of NAT:

NAT: Network Address Translator...

as per its name its function is to translate the ip lets say about private and publi ip address...

Private ip address are those addresses which you can use for you internal network and those ips are excluded in all class like class C is having range from ...this ip address are not allow in internet.

Public IP address are those addresses which are allowed in internet work. If you want to communicate with the outside in internet then you need to have public ip addresses...

so normally you are having Private ip address in you lan and having Public ip address from where your loaca lan traffic will translate to the public ip and access the internet...which is you gatway for internet...and we consider that gateway as your when router receive the traffic from local lan it will translate the local lan ip addresses in to the public ip addresses and now your local lan traffic can travel in to this process is nothing but NAT...

let say your privat ip address is 192.168.x.x and public ip address assign by your isp is gateway router will translate your loacal lan private ip address 192.168.x.x to 202.202.x.x and now your traffic will travel to internet...

so when you have pool of public ip then you just configure that pool to your gateway router and router will translate the ip when it will receive the local lan traffic...

hope this will clear your idea of NAT

rate this post if it helps



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