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Could someone please explain in general terms how I'd configure a Cisco 1841 (2x ethernet ports) to route a public /29 address block (6 hosts) to my internal network.

I wish to give the Cisco router one public IP and then several of my internal Windows servers will receive the other public IP addresses. Other hosts behind the router will access Internet via NAT.

I'm a bit confused as I've only ever setup routers/firewalls that had a single public IP address with NAT and port forwarding to internal servers.

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3 Answers 3

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How do you get your connexion to Internet ? If you have a provider modem/router that is already in this /29 you don't need a router. You can still use one but it means that all IPs of the /29 will be on the Wan port, you will need to use internal IPs for server then nat public IP to private IP.
In the other case there is an interconnection network between your IPs and your provider. This means that the Wan port on the router get an IP outside of your /29, then you juste have to configure your internal network with your /29.

So the big question is, how are you interconnected

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I'll have to check - if the router was within the /29 with all IP's on the WAN port then I could just nat each of public IPs individually to a different internal IP? –  Jonathan Jun 2 '10 at 23:25
    
Yes you can do that –  radius Jun 3 '10 at 9:00

I don't know what you're doing, but I'd give the internal Windows servers private, static IP addresses, and NAT the public addresses through to the private ones.

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It is not very common, but this can be accomplished with proxy arp. Basically you assign one IP address from your subnet to both the internal and external interfaces. Then you enable proxy arp so the router will answer ARP requests from either side.

I am a bit curious why you have a router at all? Is this so you can have a place to set firewall rules?

It may be easier, and more common to not route the unused addresses inside, and instead leave it all on the outside interface and just setup NAT.

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Yes - router was mainly to act as a firewall (and vpn endpoint) –  Jonathan Jun 2 '10 at 23:14

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