Saturday, February 15, 2014

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

This is an interesting question.

Internet Service Providers have long handed out subnets of, 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.

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

Direct Link to video:

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



  1. Video has been removed?

  2. Anonymous10:17 AM

    I don't believe we've encountered an ISP that doesn't give you a "true subnet" in our neck of the woods. That includes Centurylink, Time Warner, Comcast and even the local guys.


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