Correct way to configure 2 interfaces on 2811

Greetings All:
Weve initiated a new ISDN service to support our VOIP & data needs.  The service is using 2 Cisco routers with one configured as VOIP router & the other as WAN.  The ISP issued us 30 ip addresses for use on the data side but reserved 2 of the addresses to configure the routers.  To enable VOIP services the engineers used the interface FastEthernet0/0 on both the WAN & VOIP routers and instructed us to use FastEthernet0/1 on the WAN router to connect for data.  However when I try and assign an IP address to the FastEthernet0/1 connection I receive an IP address overlap error.

Is there a way to configure the FastEthernet0/1 connection using one of my remaining 28 ip addresses?   Or should the ISP have given me a unique ip address separate from my block of data addresses to drive the VOIP interface? Ive been researching in Experts Exchange database for similar situations & possible solutions but have come up short.

Any assistance would be appreciated.
Ken CarpenterAsked:
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hau_itConnect With a Mentor Commented:
This a typical VLSM example.
Lets say that your ISP gave you the network 200.1.1.0/27. That network has 30 IP address. In order to be able to use these IP addresses in more than one network you have to separate this network and create2 that have 14 addresses per subnet.
In my example you can use the 200.1.1.0/28 and 200.1.1.16/28

the /28 means 8 bits mask - 255.255.255.240

Dimitris
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SimWhiteCommented:
You must use others block of IP's, or you can devide your IP's in subnets by mask.
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Ken CarpenterAuthor Commented:
Greetings: Apologies for my delay in responding. Your answer was right on. Using the knowledge you provided I was quickly able to express the issue to the ISP technician who just as quickly saw the issue and corrected the mask & IP information so as not to void any support agreements with the ISP.  Thank you.  
Just out of curiosity - Is there any special kind of cable that I need to use to connect from my FastEthernet0/1 interface to my linux firewall?  I've configured the interface and tried to bring it up and the interface say's that it's up but the line protocol is down.  I've tried connecting to a linux firewall using a 568-a cable, a 568-b cable, a cross over cable, and a straight ribbon cable and can't get a link light from the router.
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hau_itCommented:
Happy to help you!!!


In order to connect a router with a workstation you have to use a crossover cable (568A - 568B)
If the link wont come up then check that you issued the no shut interface command on your router.
Also check your cable!!
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