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New Member

Ip addressing and subnets

I was wondering if anyone knew if I could use a subnet mask of with a network number of I believe I cannot use this mask with a Class C address but since this is a class A address I though I might be able to. I want to have two subnets each with 126 addresses. These address will be split between two office each with a Cisco 3620 router.


Scott Graham

  • Optical Networking

Re: Ip addressing and subnets

The topic of this forum is IP + Optical however there are a number of professionals online who may be able to assist you. We are always considering additional forums for such topics and I will make a note of your post.

If you don't get a suitable response to your post, you may wish to review our resources at the online Technical Assistance Center or speak with a TAC engineer. You can open a TAC case online at

If anyone else in the forum has some advice, please reply to this thread.

Thank you for posting.

New Member

Re: Ip addressing and subnets


There is a time to and not to use the first and last subnet. In your case, you have no choice since there are only two subnets. This applies to class A, B, & C. In short, a /25 ( subnet can be used on either class. In your case, is your first usable subnet while is your last.


New Member

Re: Ip addressing and subnets

Hi Scott, off course you can Use the 25 bits mask, I supouse you are using the a subnet of the net with 24 bits, it is true? and if you use EIGRP or OSPF or RIP2, you can use this mask, but you have to take care if you are using non cisco Routers.

But I suggest you to make a lab, and test all you need.

Jorge Hto Gonzalez

CCIE 6525

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