Re: 255.255.254.0 subnet mask vs 255.255.255.0 subnet mask

The difference is 256. That is the 255.255.255.0 subnet is 256 addresses. One is the network (.0) and one the broadcast (.255) so you have 254 usable.

Going to the 255.255.254.0 subnet the last 9 bits are usable hosts so 510 total usable + network and broadcast.

That is the simple part and you probably knew that. However, let's take two examples.

#1
V V
192.168.0.0 with the 255.255.255.0 mask means you can use 192.168.0.1 to 192.168.0.254
V
With the 255.255.254.0 mask you can use addresses 192.168.0.1 all the way to 192.168.1.254 so the following 256 are added.

#2

V V
192.168.1.0 with the 255.255.255.0 mask means you can use 192.168.1.1 to 192.168.1.254.
V
With the 255.255.254.0 mask you can use addresses 192.168.0.1 all the way to 192.168.1.254 so the PREVIOUS 256 are added.

It's a binary thing. :-)

So MY guess (if you can get him to fess up to this) is your network guy goofed and expected he was in example 1 where an additional 256 addresses would be added following but he has since discovered that he got those preceding the current network. And it's likely that subnet is in use. OOps.


- Larry "DrFranken" Bolhuis

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On 1/3/2019 2:37 PM, Rob Berendt wrote:

Network guy had me set up our DMZ based LPARs with a 255.255.254.0 mask.
Now he wants me to change it to a 255.255.255.0 subnet mask.
I told him that it would have to wait until our next scheduled downtime.
Which is in March.

I am trying to figure out why. I can't get an explanation from him that I
can understand. Knowing why might help me to determine what I should test
to see what it broke. For example, since he discovered this by analyzing
traffic I am assuming that something is using this.



Rob Berendt