Question : T1 Accessing Public IP Address's

Ok, just got a Integrated T1 line (Voice and Data) installed with an Adtran 3200 Router. The Telco company configured the router for nat and everything is working great. With the service I have 5 public ip addresses. I would now like to use one of the public IP addresses included in my block. Currently, when I check to see what public IP address all the nat clients are running out of it is the one the telco company calls the Customer Serial IP Address. Here is the setup

WAN Default Gateway - *.151.101.57
Customer Serial IP - *151.101.58
Subnet Mask - 255.255.255.252

Public Lan Network - *153.125.200
Usable IP's - *153.125.201-206
Subnet Mask - 255.255.255.248

NAT Clients for Internet Access - Default Gateway 192.168.1.1

How can I access one of the public IP addresses? I plan on puting a router behind it and running nat for a separate network. Should I disable NAT on the T1 Router?

Site Note Questions:
Why did the telco company choose to run nat behind the customer serial ip address and not a public lan address?
What is the purpose of the Serial IP addresses, why can't you interface the Default Public Lan Gateway directly to the Telcos Router and eliminate the Serial IP Addresses (If this doesn't make any sense just say so)?

Also, We have a PBX running with this configuration and I don't want to disrupt its service:

Avaya IPOffice 403 PBX - 192.168.42.2
Computer To Configure PBX - 192.168.42.1

Thanks in advance.

Answer : T1 Accessing Public IP Address's

>Why did the telco company choose to run nat behind the customer serial ip address and not a public lan address?

Probably because most customers don't use their allocated IPs, and the NAT/DHCP combo tends to make things just automagically work for folks that don't care.

>What is the purpose of the Serial IP addresses

It's probably the router's external interface IP.

>why can't you interface the Default Public Lan Gateway directly to the Telcos Router and eliminate the Serial IP Addresses (If >this doesn't make any sense just say so)?

You *can* run protocols (ie HDLC) that support unnumbered interfaces (interfaces that have no IP address), but an unnumbered interface doesn't lend itself well to the aformentioned NAT/DHCP combo config (you tend to need an external IP address to translate to).

Your solution may be as simple as performing the folling steps (assuming some of them have not already been performed by your telco/ISP):

1.  Assign one of your public IPs (*.153.125.201-206) to the Adtran internal ethernet interface (assuming it does not already have an assigned public IP address - I wonder since your telco told you you have 5 IPs, but I count 6 [201-206]).  You should likely turn off DHCP and NAT on the Adtran.  The public IP assigned to your Adtran internal ethernet interface will be entered as the default gateway for whatever router/firewall you put behind the Adtran.

2.  Assign another of your public IPs to the router/firewall you intend to install.  Set the default gateway to be the IP of the Adtran.

3.  Configure your router/firewall to hand out DHCP for 192.168.42.x (avoid the IPs for your PBX and PBX PC).  The PBX and controlling PC should continue to work fine.

Cheers,
-Jon