+ Post New Thread
Page 2 of 2 FirstFirst 12
Results 16 to 18 of 18
Wireless Networks Thread, Extending our current ip range in Technical; It's a little (okay, a lot) complicated, but to determine what's a "local" address to you, XOR the destination against ...
  1. #16
    oldbaritone's Avatar
    Join Date
    Feb 2009
    Location
    Binghamton, NY
    Posts
    5
    Thank Post
    3
    Thanked 2 Times in 2 Posts
    Rep Power
    0

    Net Masks, Subnets, and Gateways

    It's a little (okay, a lot) complicated, but to determine what's a "local" address to you, XOR the destination against your own address (i.e. find out what bits are different) and then AND that result with the net mask. If the result is zero, it's a local address; if not, send the request to your gateway.

    dsk, in your case, a student, let's say 10.64.92.123, tries to access an admin server at 10.64.90.1: (It's a lot easier to see this in Hex)
    0A.40.5C.7B (student IP 10.64.92.123)
    0A.40.5A.01 (admin IP 10.64.90.1)
    00.00.06.7A (result of the XOR - just an intermediate result)
    FF.FF.FE.00 (255.255.254.0/23 Net Mask - AND against the above)
    00.00.06.00 (result is nonzero, so dest is non-local and request goes to gateway)

    Now, do the same thing with the 248.0/21 Mask:
    0A.40.5C.7B (student IP 10.64.92.123)
    0A.40.5A.01 (admin IP 10.64.90.1)
    00.00.06.7A (result of the XOR of the first two)
    FF.FF.F8.00 (255.255.248.0/21 Mask - AND with above)
    00.00.00.00 (result is zero, so request is local and direct communication is used)

    In the first example, the request will be sent from the curri address to the gateway, which probably has IPsec rules to block traffic between the subnets. End result, the student in the curri subnet can't get to the admin server.

    In the second example, the request will be processed directly between the two clients. There will be no gateway involved in the request, so the student will be able to communicate with the admin server. This amounts to plugging all of your students onto the admin subnet - probably not a good idea.

    My suggestion would be to move one subnet farther away from the other subnet - at least to 10.64.128.xx if you can. The RFC1918 subnet reserves ALL of 10.x.x.x, so you might even want to look at moving the subnet block even farther - like 10.65.x.x, which would give you 65,535 addresses for each subnet. I'd guess it's probably easier to move the curriculum subnet, which is probably straight DHCP? Then you can have more space (i.e. use the 255.255.248.0/21 mask, or go to 255.255.240.0/20 - you can even go all the way to 255.255.0.0/16) without combining the subnets.

    krb548 - your "supernet" is really the same thing, the other way around. The "standard" subnet for 192.168.x.x is 255.255.255.0, which gives each client direct access to any address that has the same first 3 numbers in the IP, or a subnet of 254 addresses. Your "supernet" is just a larger mask access for local communication.
    "Good news-Bad news:" You're not going to be able to expand to 192.168.9.x, because that doesn't work in Binary/Hex.
    Your choices will be:
    192.168.0.? - 192.168.0.255 (Net Mask 255.255.255.0)
    192.168.0.? - 192.168.1.255 (Net Mask 255.255.254.0)
    192.168.0.? - 192.168.3.255 (Net Mask 255.255.252.0)
    192.168.0.? - 192.168.7.255 (Net Mask 255.255.248.0)
    192.168.0.? - 192.168.15.255 (Net Mask 255.255.240.0)
    (the Binary/Hex is left as an exercise for the student...)

    ;-)

    (Tech note: some 192.168.x.x routers may have troubles with the ".0" and/or the ".255" addresses because they don't handle the subnet properly according to the spec. I'd suggest avoiding ".0" and ".255" address assignments in the 192.168 block because of addressing problems in cheap routers. And the "supernet" may or may not work in 192.168.x.x for the same reason - If it doesn't, go to either the 10.x.x.x subnet, or the 172.16.x.x-172.31.x.x subnet, which are also reserved addresses as specified in RFC1918)

  2. Thanks to oldbaritone from:

    SimpleSi (19th February 2009)

  3. #17
    Jay
    Jay is offline

    Join Date
    Mar 2008
    Location
    Autocratic theocracy of Norfolk
    Posts
    71
    Thank Post
    3
    Thanked 4 Times in 4 Posts
    Rep Power
    14
    For info:

    Our supplier informed me that CISCO recommended broadcast domains of no more than 1024 hosts.

  4. Thanks to Jay from:

    oldbaritone (20th February 2009)

  5. #18
    oldbaritone's Avatar
    Join Date
    Feb 2009
    Location
    Binghamton, NY
    Posts
    5
    Thank Post
    3
    Thanked 2 Times in 2 Posts
    Rep Power
    0

    Subnet limitations.

    Good point, Jay.

    The previous posts were wanting 2,000 - 4,000 or more addresses in a subnet. Certainly by the time the address space has gotten that large, it's time to move away from flat topology and subnet the address space (like admin or curriculum) into smaller broadcast domains, like department or building or dorm.

    My explanation was about mechanics of the protocol, and certainly was not a recommendation to make domains that large. But if someone insists on being foolish, that's their prerogative.

    Thanks.



SHARE:
+ Post New Thread
Page 2 of 2 FirstFirst 12

Similar Threads

  1. Replies: 8
    Last Post: 15th December 2008, 04:41 PM
  2. Promethean WB USB cable extending
    By sbutterworthtj in forum Hardware
    Replies: 6
    Last Post: 18th November 2008, 01:14 PM
  3. Extending IP range HELP!!
    By gh256 in forum Wireless Networks
    Replies: 2
    Last Post: 12th May 2008, 12:11 PM
  4. Extending IP Range
    By Grommit in forum Windows
    Replies: 12
    Last Post: 17th November 2006, 05:59 PM
  5. Extending a Raid
    By Kyle in forum Windows
    Replies: 10
    Last Post: 1st November 2006, 08:35 PM

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •