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Extend usable IP address range (external IP)

    I am asking this question on behalf of our client
They have 2 offices (HO and a branch office) and are currently using Class 'C' external IP range for all their PCs and servers (They dont seem to know the reason why they are using external IP range) ( to SNM : for HO and to, SNM: for their branch office. (the two sites are connected using fibre connection)
         The DC is Windows 2000 advanced server. All servers are assigned Static IP and based in HO. DHCP  scope is used to automate the IP Addressing in client side. DHCP and DNS services are running.on DC.  A exchange server is running for email purpose. HO servers includes domain controllers, PC, Teklogix,Hand Punch Machine,cameras & printers.As their network is expanding, they want to get more IP address (atleast another 254 addresses)  
AS  they are 24 x 7 hrs operational, they would like to accomplish a solution with minimum down time and that gives them Atleast another 254 IP address. Could some one explain in details what needs to be done to complete this project  smoothly.?  At the moment they have a layer3 switch  and would like to do them selves rather than doing config changes from ISP side


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Oh god.  You have nightmare on your hands.  Without changing anything, they are going to need to put in a router and create a new internal class C segregated from their current network and then route that back to the current network.  Now, that said, smack your client upside the head and propose that they Rebuild the entire network using internal address space and then get rid of the Full Class C for the External range.
Fred MarshallPrincipalCommented:
Unless the DC arrangement creates some unforeseen difficulties, here is an assessment and a probable solution:

The client is using a single subnet and has simply divided the two office address spaces by convention.  All computers throughout are on the same subnet.  So that keeps it as simple as can be.
The client has chosen a public IP address space unfortunately - but this may not matter all that much.
You don't tell us how the internet connection(s) are made or if there are any actual "public" servers in this operation.  Actually I'd have serious concerns if there *IS* an internet connection - how do you separate the "LAN" out from the outside world in that case????  How do they firewall???

Here is what I'd do - *at least as a thought experiment*:
Select the address range /  That gives them plenty of private addresses.
If *everything* is set up to get addresses via DHCP then that makes it pretty simple.  Look out for the few hosts that have manually entered IP addresses.  In fact, you may want to check this out first to confirm DHCP is being used in critical hosts or in all hosts.

Then, when you're ready, change the IP addresses of the DHCP servers and reboot all the computers they serve (rebooting is simpler to describe than other methods that might be used).
Other than My Network places, the connectivity should be immediate.
My Network places could take a while to figure itself out.
If you're using WINS or some such then you may have to pre-configure tables to suit the new environment before making the switch.  
This would be the area to watch ... how do the hosts get name service for the network?  You don't want to "break" that function.  Presumably the DC takes care of this for you.

When done, all the computers can talk to each other as before - just with a new set of IP addresses.  

What remains to discuss is the internet connection.  Since that's not described then one might only imagine......

The simplest thing to do is to introduce a router that will handle the traffic and plug it in between the internet access point (modem, fiber, whatever) and the LAN.  Have NAT turned on (the usual default) and set the LAN address to match the LAN subnet address space.  You will probably have to manually set up the LAN address as it's a bit uncommon for routers to take LAN addresses via DHCP.

Much has not been said but this would seem to be a roadmap.

He mentioned that they do have Exchange, so there is an Internet connection which is the scary part.  This means that the Defualt gateways and any static routes would need to be changed as well as the firewall/possible NAT configurations (please tell me there is a firewall).  You plan is sound, but as far as the static addresses go for PCs and Servers (exception Active Directory Domain controllers -- if any) would be to Register the MAC Addresses in DHCP  as reservations that never Expire and change all the servers to DHCP.  This keeps the addresses centralized in a nice database for management and allows for easy moving/re-IP of the systems.  I know a lot of people frown on using DHCP for servers but it works great, just not your AD boxes or clusters.... Just set them manually and still put the addresses in DHCP.
ExchangeGroupAuthor Commented:
I haven't had the full information so nothing to blame the expert. I handed over the suggestions to my client and they will decide what needs to be done.
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