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Dual IP Addresses on the Same Segment?

By MLivingston ·
Anyone have experience with this strategy? I have 2 non-contiguous class C addresses, it's been suggested that I use both on the same physical network segment, with a 2-NIC router attached as well. Each NIC will need its own unique address from one of the Class C addresses - and the routing function would allow the two networks talk to each other. I'd have never thought of this - and wonder just how commonly used this idea is.



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by JasonC In reply to Dual IP Addresses on the ...

Another way of doing this is to put a "Secondary" IP address on the Ethernet of the router. If it's a Cisco, just add the following under Interface Ethernet0
"ip address secondary"

That way you only use one interface;

Other than that, there's no problems doing it! You can in fact have several secondary IP addresses in this way; the only potential issue is the bandwidth available on the network.

We've done it at a number of our sites during IP address transition; eg running the old and the new side-by-side, so we could move clients and servers etc. over one at a time.


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Secondary IP on NT 40 box

by dlambert In reply to

Hope you can help. I had a configuration set up like this on a message switch for a mobile network. (Police Department Mobile laptops.) The Message switch a NT40 SP3 machine had two ip's on the same NIC card. and with a default router of the default router had routes to handle 198.228.31.X to which was the router for the mobile frame relay connection. Laptops all on the 192.228.31.x segment. also had a secondary ip of on the E0. Last week the vendor for the message switch came in and blew up the server loading service pack 5. After reloading he could not get the dual ip addres sytem to communicate with the mobile segment. 198.228.31.x. Can you hopefully give me an idea of what hey screwed up. We had to add address translation on the router to get around the problem. Any Help I would greatly appreciate.

Dennis L.

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by JasonC In reply to Secondary IP on NT 40 box


(Sorry for the delayed reply)

Assuming that the "Source Address" of the NT Server needed to be in the 192.168.241.x range in order to be carried over the Mobile Data Network, this should have worked Ok provided that the subnet mask's were consistent (both at a guess), as the NT Box would use it's interface in to that subnet to talk to the other devices.

The NT box would also need a manual static route to the 198.228.31.x network via Otherwise it'd probably try for a route via the default GW.

Hope this helps...


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Yes, but ...

by eBob In reply to Dual IP Addresses on the ...

You will have a lot of collisions on your Ethernet segment which your users will detect as various performance problems.

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Thanks a bunch for all the input!

by MLivingston In reply to Yes, but ...

I have a switched network - so as long as my switch can handle the load, collisions shouldn't be an issue (as I understand it). I am using public addresses (assigned to me via the Network Information Center), and most users will probably need to beindividaully identified according to the various web sites they access. I noticed the tcp newsgroup had several messages on this same issue.

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Dual IP Addresses on the Same Segment?

by dmoufarrege In reply to Dual IP Addresses on the ...

You can do that. However, are these public IP addresses, or are you using private (reserved) addresses?
If they are public addresses, does each machine need a public address? If they are private, what prevents you from using a class A or B reservedblock?

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Using a Private IP Address

by MLivingston In reply to Dual IP Addresses on the ...

Before I was exposed to the concept of running several IP addresses on one physical network - that's exactly what my strategy was to be...but that would require physically changing every workstation IP address (unless you know of an automated way).If I could simply add addresses from the barely used IP address as we gain more PCs on the network - I don't need to backtrack anywhere.

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by dmoufarrege In reply to Using a Private IP Addres ...

How about installing a DHCP server on the network and rolling out all new PCs w/ dynamic configurations. That allows you to keep all existing PCs as they are and make changes as you do other things to the machines (such as upgrades and replacements).
The added advantage is a central point to change gateway information and the like.

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