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We are currently working on migrating a lot of new machines to our network, and my job this morning was to setup all of the IP Addresses. I worked on this all morning, and when I got back tonight I was informed that they had all been setup incorrectly, and had to be removed and re-added. I am quite confused as I have been setting up IP's on machines for a long time and I am curious as to what the issue is.

Just taking into account this example...

72.26.196.160/29 255.255.255.248

A /29 block is 5 usable IP's. With the script I wrote and used, the IP Addresses .162 - .166 were added to the NIC. I can't remember now what the name for .161 was, but isn't it the broadcast address or something which isn't assigned to the NIC when adding additional IP Blocks?

I am curious as to where my logic is failing me. Not to mention even if .161 was to be added, there is no reason why all of the IPs would have to be removed, as .161 could just be added in addition to these.

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1 Answer 1

up vote 2 down vote accepted

Your logic is failing you. 72.26.196.160/29 provides for 6 useable ip addresses:

72.26.196.161 through 72.26.196.166

72.26.196.160 is the network address (Subnet ID) and 72.26.196.167 is the broadcast address

Aside from that, your question doesn't really make sense. Can you provide some additional detail and context?

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Thank you for the information. I am thinking the case may be that I just learned wrong when I started working here years ago. Here is a machine that was setup well before my time in the same way. 96.44.139.128/26 and the first usable IP assigned is 96.44.139.130. The .129 is used for the Gateway then the first usable is .130. –  Brett Powell Apr 5 '12 at 2:25
    
So I guess my flawed logic just came from my previous employer, although even when doing a Google search I see numerous places that say that a /29 is a block of 5 usable IPs. –  Brett Powell Apr 5 '12 at 2:26
    
Your logic is not that flawed. /29 does provide 6 usable addresses, but usually the first one is used for default gateway which does leave 5 of them to be assigned to server(s) in that block. It all depends on particular setup and what did you want to achieve after all. –  dusan.bajic Apr 5 '12 at 8:05
    
Brett: I see what you're getting at. The first usable address is assigned to the DG (by convention, not by rule) leaving 5 addresses for internal hosts. I just always look at the number of total usable addresses for host assignment, the DG being one of those hosts. –  joeqwerty Apr 5 '12 at 9:38

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