Routed IP different from assigned subnet

September 16, 2009 at 06:41:02
Specs: Linux i386
I'm not a network prof but neither complete newbie. I'm have to change the configuration of the internal network since the ISP can no longer guarantee my current 3 public ip. ISP proposed me to assign a subnet.
Now the question. The strange thing is that the routed IP is not part of the subnet. I would expect that the firs usable ip from the subnet should be the routed ip.... Any idea? It is standard configuration?

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September 16, 2009 at 08:05:11
What you write doesn't make sense.

Public ips have nothing to do with internal [private] ips. Changing public ip does not affect private ip hence why your post doesn't make sense.

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September 16, 2009 at 09:19:05
The subnet is not properly private. I should have servers running on a DMZ zone on this subnet. And ISP is going to modify my DNS records. The standard configuration at least from what i know is to have the first ip of the subnet as public ip and entry point for the subnet.
The fact is that my ISP allocated me an ip from a totally different network that seems private to. It is a /25.
Any idea?

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September 16, 2009 at 09:45:19
Again with the lack of clarity.

There is public ip and private ip. There is no such thing as properly private ip.

"The standard configuration is to have the first ip of the subnet as public ip and entry point for the subnet."

Sorry but that is not how this works.

What ip are you being assigned?
What ip are you using internally?
[you can edit the second octet to xxx to hide the real ips]

/25 as in subnet mask has nothing to do with if public or private ip.

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September 16, 2009 at 12:33:43
Sorry if i'was't clear enough. I try to explain better the situation. They assign me one routed ip and one public subnet to host various services that are from completely different networks.
On every piece of documentation they appear to be from the same subnet.
The transport ip being the first subnet ip.

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September 16, 2009 at 13:07:58
Sorry I can't help you without seeing the ips/subnets. Best of luck.

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September 17, 2009 at 07:48:14
I think you are missing something very important. NAT, where is your NAT. Most networks are setup with one or more Public IPs (Static or Dynamic) which are NATed to a Private network. This gives you the ability to hook up many computers up to 254 if you use /24 like said above. If you want to go beyond 254 computers then you subnet using the subnet mask.

What you have said above seems to state that you are not using NAT at all and you are assigning the Public IPs direct to the computers. If that is the case then you ISP will give you a CIDR block which will be like /30 (CIDR Notation) or Subnet. This gives you 4 Public IPs.

"The fact is that my ISP allocated me an ip from a totally different network that seems private to. It is a /25."

If you ISP gave you this then chances are they are not private. Look at the first two octets If they are either one of these...

172.16.x.x - 172.31.x.x

Then they are private, if not then they are Public. The /25 says that you have a CIDR block of 128 address assigned to you. This means that if your first address is then you can use address up to

So you put your Static Private IP and the Subnet Mask settinging into your NIC and you are on the Internet. Then you either get your ISP or third party like GoDaddy to register your IP on the Public DNSs. This will give you a, .net, .gov or what ever the DNS you registered with. This way people can connect to you servers with the DNS name and not the IP.

Now it sounds like you are switching from Static to Dynamic IPs. If you do so then you will not be able to do any web hosting from your servers unless you get a third party proxy which will install an app on your servers and phone home to the static servers what your IP is. Its better to have static IPs from your ISP if you want to do this.

That is the quick and dirty on networking and now you can see why every one is confused because what you have said does not seem to fit. Yes there are more details than what I listed here and I am sure I am going to get flamed but you need to give us more to go on.

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