Best way to subnet a /24......

I have been given a /24 network (64.x.x.0). I would like to be able to subnet this so that I can have one network with 25 ip's (/27) and the rest /30 networks. I have a cisco 7200 that is at 64.x.x.1 but I do not control this. Basically it is my isp gateway. What is the best way to accomplish this?


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On 13.01.2007 00:53 mlc wrote

Unless you say what your optimisation parameters are it's hard to say what you mean by "best way" ...

Well, one way would be

/27 64.x.x.0/27 64.x.x.0-64.x.x.31 /30 64.x.x.32/30 64.x.x.32-64.x.x.35 /30 64.x.x.36/30 64.x.x.36-64.x.x.39 ... /30 64.x.x.252/30 64.x.x.252-64.x.x.255

To get all possible partitions, just "shift" the /27. I.e. next partition would be

/30 64.x.x.0/30 64.x.x.0-64.x.x.3 ... /30 64.x.x.28/30 64.x.x.28-64.x.x.31 /27 64.x.x.32/27 64.x.x.32-64.x.x.63 /30 ...

etc. etc. The last one is

/30 ... /30 64.x.x.220/30 64.x.x.220-64.x.x.223 /27 64.x.x.224/27 64.x.x.224-64.x.x.255

Arnold, AN45

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Arnold Nipper

If the 64.x.x.0 that you want to subnet is the same network as for the 64.x.x.1 on the 7200 that you do not control, then unless you can get some cooperation from those who control the 7200, you cannot subnet *at all* (at least not without adding more hardware.)

No matter what kind of subnetting you want to set up, you are not going to be able to overcome the fact that the 7200 is set to

64.x.x.1 and that the 7200 is using the broadcast address 64.x.x.255 .

And to get the various subnets to talk to anything other than within each isolated subnet, you need a router to move traffic between the subnets; that router is going to have to be the 7200.

It sounds as if you have the common situation where the ISP is handing you a /24, and the ISP is providing the equipment, and you have been an IP for the router within the /24 and that the ISP has an address in the /24 for the router at their end of the link.

In situations like that, one way is to get the ISP to do the subnetting.

The approach that is taken when you want to be able to control the subnetting yourself, is that the ISP gives you a public /24, and then puts a small "carrier" subnet such as a /29 or /30 between their end of the link and your end of the link. The ISP then configures so that *all* traffic for your /24 is sent to the appropriate IP in the shared /30, and you configure your end so that the default gateway is the ISP's IP in the shared /30. ISPs encounter this kind of situation quite often, and if you have a /24 from them, they probably won't even charge you extra rental for the /30 "carrier subnet" (but they might charge a nominal "installation fee" to cover their employee time.)

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Walter Roberson

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I'm not entirely clear on what you mean.

If the ISP controls your router and does not cooperate in the subnetting, then if you go ahead and subnet anyhow without adding an internal router to handle the subnets, then your outgoing (to the ISP) traffic will simply fail, because each of the subnets is going to send out ARP queries for the gateway IP, sending to the -subnet's- broadcast IP address... and the router will only be listening for those on -it's- broadcast IP address, 64.x.x.255 .

If your ISP will handle the subnetting for you and your question is whether it is a good idea to subnet down to /30's, I would say that it depends a lot on what you are trying to do. If you are using Windows, then recall that -each- subnet will want to elect a PDC and BDC. With a /30, all you have room for is the network base address (reserved), the broadcast address (reserved), the router's IP in the subnet (unusable for your purpose), and 1 user IP address... which means that every Windows machine is going to want to elect itself as PDC. Sounds like trouble to me.

Subnetting is done for broadcast control (lower network traffic) and for security. Are you in a situation where each device needs to be secured from each other? If so, then you want a firewall for the security, not a router that isn't under your control. If you are concerned about broadcast traffic, then are your devices really broadcasting (or multicasting) so much that it will help to move the intra-host traffic bottleneck over to be the router?

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