Can I Use All Eight Addresses in my 248 Subnet, or Just Five?

By Karl_Palachuk
In Channel
February 15, 2014
0 Comments
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Karl Palachuk / Karl Palachuk

This is an interesting question.

Internet Service Providers have long handed out subnets of 255.255.255.248, which mathematically calculates to eight IP addresses. At the recent SMB TechFest in Orange County I was making a presentation on TCP/IP and I got asked this question:

How Come I Can Only Use Five of the Eight IP Addresses Assigned to Me?

Well . . . for the most part, you CAN use all eight IP addresses. There was a time, a long time ago, when it was discouraged. But you’ve always been able to do it.

Having said that, it used to be the case that routers needed to be configured to allow the use of all-zeros and all-ones subnets. But ever since Cisco IOS 12, the default is that you can use these address. See http://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/13711-40.html (an article from 2005 – so this is nothing new).

I updated my slide deck to explain what’s going on here. This is a quick video with the new slides added:

Give it a gander and let me know if you have comment.

🙂

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All material Copyright (c) 2006-2013 Karl W. Palachuk unless otherwise noted.


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