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  1. #1
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    Use of .0 in IP addresses

    Just wondering, and I've not seen this documented anywhere...

    Say you have 192.168.0.0/23 ...

    Why does nobody ever make use of 192.168.0.255 and 192.168.1.0 ... I've never seen it done before and to me there doesn't seem to be a reason why it shouldn't be done.

    Dan

  2. #2
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    We have tried, but certain users could not access the IP and certain software programs had issues with it. It just seems that some software is made to think that x.x.x.0 is not a usable IP making it then difficult to actully use for much of anything. Maybe you can use it for internal use of some kind, but allocating it to a customer or using it for any public use may cause some issues.
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  3. #3

    optimum online

    Afaik cablevision ool uses .0 ip's throughout some parts of their residential network but I don't believe they assign them to end users. Cisco will let you do it afaik its just another option. They have plenty of address space though so there must be another reason that I don't know of.
    crisper

  4. #4
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    He's not talking about "ip subnet-zero", which is the Cisco command to allow use of the network address in a range. Those are indeed valid IP addresses for a /23. We have /22s for our TSM backup network segments, so .0 and .255 get used in every single segment that exists in our data centers.

    It doesn't surprise me that some applications would have issues with those addresses though; most developers are oblivious to how networking works. Just this week I had our Oracle SE telling me that I could improve our 500 microsecond (0.5 ms) latency between metro sites by "turning on an option in the DWDM switch". I'm still waiting for him to get back to me on that one; I've looked and couldn't find the "observe_speed_of_light=no" option.
    Last edited by spaethco; 10-29-2006 at 02:37 PM.
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  5. #5
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    Quote Originally Posted by spaethco
    Just this week I had our Oracle SE telling me that I could improve our 500 microsecond (0.5 ms) latency between metro sites by "turning on an option in the DWDM switch". I'm still waiting for him to get back to me on that one; I've looked and couldn't find the "observe_speed_of_light=no" option.


    I'm sorry but I found that rather funny.

  6. #6
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    The software that Karl is probably referring to (among other things) is Windows, cPanel, and DirectAdmin. I believe in the case of DA, you can use the first IP of a given subnet range but not the last. But the bottom line is that I only count on .0 and .255 working in vanilla Linux. As has already been pointed out, it's probably not a good idea to give these to your customers.

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  7. #7
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    If you look at how IP is designed to work, on every network and subnet, two addresses are reserved.

    At the low end of the range of addresses for the network or subnet, xxx.xxx.xxx.0, is the address for the network or subnet itself.

    The address at the high end of the range of addresses, xxx.xxx.xxx.255, is the broadcast address. Any message sent to the broadcast address will be received by every host on the network.
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  8. #8
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    Quote Originally Posted by NyteOwl
    If you look at how IP is designed to work, on every network and subnet, two addresses are reserved.

    At the low end of the range of addresses for the network or subnet, xxx.xxx.xxx.0, is the address for the network or subnet itself.

    The address at the high end of the range of addresses, xxx.xxx.xxx.255, is the broadcast address. Any message sent to the broadcast address will be received by every host on the network.
    That's where you're wrong, and this is the point in me making the post.

    In a /24 this is indeed the case, but in bigger blocks you have .255 and .0 in the middle of the block that technically could be used, but never seem to be ...

    It doesn't surprise me that some applications would have issues with those addresses though; most developers are oblivious to how networking works. Just this week I had our Oracle SE telling me that I could improve our 500 microsecond (0.5 ms) latency between metro sites by "turning on an option in the DWDM switch". I'm still waiting for him to get back to me on that one; I've looked and couldn't find the "observe_speed_of_light=no" option.
    LOL, nicely put.

    In all seriousness thanks for the input, I had a feeling that would be the case, was more curious if anyone else out there was actually using those addresses sucessfully.

    Dan

  9. #9
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    Quote Originally Posted by tical
    The software that Karl is probably referring to (among other things) is Windows, cPanel, and DirectAdmin.
    Add HSphere to that list.
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  10. #10
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    Windows will not recognize .255 as a valid IP on /23 or greater subnets.
    http://support.microsoft.com/kb/281579/en-us. I don't specifically recall if this this applies to Win03 or not, but I believe that it does.
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  11. #11
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    Quote Originally Posted by Jay Suds
    Windows will not recognize .255 as a valid IP on /23 or greater subnets.
    I'm pretty sure that's been fixed for a few years now. I have Windows 2000 (SP4) and Win2003 servers on my network right now that have addresses that land on .255.
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  12. #12
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    H:\>tracert 192.168.4.0

    Tracing route to 192.168.4.0 over a maximum of 30 hops

    1 <1 ms <1 ms <1 ms 89.145.70.195
    2 <1 ms <1 ms <1 ms 192.168.4.0

    Trace complete.
    192.168.4.0 is a Windows 2003 box

    Also just tested it on my old desktop which is Win 2000 SP4, and it works properly.

    -D
    Last edited by dkitchen; 10-31-2006 at 07:29 PM.

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