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.
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: http://youtu.be/G6Nbr56L4a8
Give it a gander and let me know if you have comment.