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Subnetting a Class C Address - need help

By TomSal ·
Somewhere along the line I must of forgetten my rules for subnetting, or perhaps I enjoyed my recent vacation too much. At any rate this is situation I'm in right now...

My 100 node network is current just a static class C address ( We have telecom installers for a new VOIP system coming Wednesday and they asked me to provide them a full 254 ip's (one full subnet). I didn't think this would be a problem, I've done this before in the past. However, two fellow employees (who are higher than me on the corporate organization chart) are possessed with the notion that I can simply subnet a class C and therefore have two subnets (the original 192.168.1.x plus the "new" one) and BOTH will provide for 254 addresses.

Now I've done my little subnet scribbling on a piece of paper and I double checked with a subnet calculator -- there's no way I'm coming up with a CLASS *C* subnet strategy that provides for *2* 254 address subnets. (I show 2 subnets at 126 each).

Am I'm confused or insane?

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by pgm554 In reply to Subnetting a Class C Addr ...

A class C network can have no more than 254 unique addresses (0 and 255 reserved).

If this is not a public IP range (and I don't think it is), then you can use 192.168.2.x as a range, and then you'll just have to adjust your gateway strategy.

Your only public IP address probably comes from your ISP.
Everything else is just probably NAT'd.

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by TomSal In reply to

Thanks guys and/or gals for your input. I was thinking way too hard on this one for my own good. I was reading into "subnetting" Class C, when all the requirements state is that the need THEIR OWN subnet. Hence as stated by pgm554 the 192.168.2.x strategy is all I need to do. I was having a brain freeze of huge proportions. Thanks to both of you for showing fast interest in supporting my issue and for your excellent information.

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by Deadly Ernest In reply to Subnetting a Class C Addr ...

Answer to your situation:

As previously stated you can set up a whole new class C subnet and link them through a gateway But you cannot set up two subnet with the same IP addresses on the same gateway. The other subnet would have to be a different IP address range.

192.168.0.x is the class C 'reserved for internal use' ip address ranges, as are 172.1.x.x class B and 10.x.x.x class A.

In the long term your best strategy would be to change over to using the class A or class B reserved address range and restructure your existing address and then incorporate the new, and future, expansion within the new structure.

Thus you could have 172.1.2.x for your existing subnet and use 172.1.3.x for the new subnet whilst using 172.1.1.x for your gateway area and this would leave you plenty of room for later expansion whilst still staying within the same class group and allowing for easy administration.

If you do NOT want to make any changes to the existing subnet, then just use 172.1.2.x for the new subnet (leave 172.1.1.x for corporate gateway later) and set up a gateway between the subnets linking 192.168.0.x with 172.1.2.x

Answer to your question:

You can subnet within a class C but you get less available hosts between both subnets, the total number of addresses does not change but each subnet will need it own broadcast address etc. Thus reducing the total available for hosts.

Subnets are created on the binary code, thus many need to be larger than needed to fit the next binary adjustment. Sned me an e-mail and I will send you more extensive data, but you don't need it for this.

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by TomSal In reply to

Thanks. See Above comment. :)

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by TomSal In reply to Subnetting a Class C Addr ...

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