IP Subnet Mask Advice Sought

Discussion in 'Home Networking' started by George, Jan 22, 2006.

  1. George

    George Guest

    Hi,

    What exactly is a SUBNET, and what is it used for?

    I have just noticed that the subnet mask of my router is set to
    '255.255.255.0', but the subnet settings in a couple of computers connected
    to my wireless network are set to 255.255.0.0

    Although I have thus far not noticed any problems with accessing any
    services on the network, what is the likely problems of having the subnet
    not matching between the computers and the router? Advice on this would be
    very welcome.

    I'm a little bit confused about subnets altogether, and I'm not sure how
    they need to be set, or what effects I would get if I don't have matching
    subnets, so any advice on subnets, and how they work or have to be set up
    would be very much appreciated.

    Thanks a lot
     
    George, Jan 22, 2006
    #1
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  2. to divide the IP address into a part that identifies the network (or
    sub-network) and the individual machines.

    255.255.255.0 means only the last part of the IP identifies the
    machine

    255.0.0.0 means the last three parts, and so on.

    255 in binary is 11111111 and this is used as a mask so 255.255.0.0
    means 11111111.11111111.00000000.00000000

    think of the zeros as holes in the mask through which you can see a PC

    What difference does it make ?

    take two IP s - 192.168.0.1 and 192.168.1.1

    if the mask is 255.255.255.0 these are on two different subnets and
    would need a router to communicate. If the mask is 255.255.0.0 they
    are on the same subnet and can chat happily.

    Phil
     
    Phil Thompson, Jan 22, 2006
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  3. George

    George Guest

    So am I right in thinking, that following this rule, if the subnet mask was
    255.0.0.0, then the following IPs would be able to chat happily too?
    192.168.0.1 - 192.167.0.1 - 192.168.1.1?

    Do I have that right Phil??

    So is there any problem in running a subnet mask of 255.255.255.0 on my
    router (on 192.168.0.1) and running a subnet of 255.255.0.0 on the computers
    connected to it?

    Am I right in thinking that if all your network IP addresses follow each
    other, like 192.168.0.1, 192.168.0.2, 192.168.0.3, 192.168.0.4,
    192.168.0.5, etc etc, then the subnet mask needs to be just set to
    255.255.255.0 ????

    Thanks Phil.....

    George
     
    George, Jan 22, 2006
    #3
  4. George

    Stroller Guest

    My name's not Phil, but you have that correct.
    That would be unusual - there may be reasons or doing something like
    that, but more likely it's "incorrect" behaviour.

    If the router is 192.168.0.1/255.255.255.0 and a PC
    192.168.1.1/255.255.0.0 then the PC will think it's on the same subnet
    as the router (and will send packets to it) but the router will think
    it's on a different subnet and be unable to send packets to the PC (so
    will try to gateway them through the ISP's router, which will fail).
    Yes. Normally so.

    Stroller.
     
    Stroller, Jan 23, 2006
    #4
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