What is the 3rd group of #'s in an IP address and why do they gotta be the same

Discussion in 'Wireless Networking' started by Robert Blackwell, Dec 27, 2004.

  1. Do each of the 4 groups of numbers in an ip address have their own name?
    Particularly the 3rd group.

    If computers on a network dont share the same 3rd group number they wont see
    each other on a network, or access the internet if that number doesn't match
    up with the one your router is on. I'm just wondering about this
    Robert Blackwell, Dec 27, 2004
    1. Advertisements

  2. Robert Blackwell

    Dave Guest

    not that i have heard of. and there is nothing magic about the 3rd octet in
    the address. the key is the 'subnet mask'. all of the bits included in the
    subnet mask have to match for two devices to talk directly to each other.
    normally the subnet mask defaults to which means that the
    first 3 octets have to match for the two devices to talk. but if the
    subnet mask is then only the first 2 have to match.

    so if the subnet mask is the following groups can all talk to
    each other: through through through
    but if the mask is then the following ranges work: through through

    you have to think binary for finer breakdowns... a mask like
    gives ranges like: through
    or a mask like gives ranges like: through
    Dave, Dec 28, 2004
    1. Advertisements

  3. Okay, that makes sense and helps a lot.

    See, I just added wifi to my current network but the routers ip is like and it would dhcp any wifi defices to 2.x so the hard wired
    computers couldn't see em, I had to manually set em to a static ip of .0.x

    So, now it's my understanding that if I configthe lan ip of the wifi router
    to a subnet of any wifi devices it dhcp's will still be able to
    access the main network, but not the otherway around?

    Most impotant for me is just being able to have the wifi devices access
    resources on the network which is why I had to static them to 0.x
    But our primary router doesn't show the wifi router as an "attached device"
    so I can't configure further it either without changing my ip.
    Robert Blackwell, Dec 29, 2004
  4. Robert Blackwell

    Dave Guest

    you probably have a conflict with two dhcp servers on the network now. you
    don't want to mix subnet masks, that won't work either. with what you
    describe the wireless devices will be able to send to the lan devices, but
    the lan devices will try to reply through their router as if it was an
    outside address which apparently your routher can't handle. the best way to
    fix it is to disable the wireless router's dhcp server service and let the
    lan router handle all the ip's so they are all on the same subnet.
    alternately you could play with manually configuring routing tables and
    manually assigning ip's, masks, gateways, etc... but i wouldn't want to try
    it. i just had a friend go through a similar mixup and it can be a
    confusing situation to try to sort out unless you are real familiar with
    tcp/ip addressing and all the device configurations
    Dave, Dec 29, 2004
  5. Another way to think about it is that any traffic directed anywhere other
    than the current subnet will be sent to and through the WAN port of the
    router/gateway. Thus, if you us a subnet mask of, your IP
    address is and you are trying to reach, this
    will be sent to the router/gateway and then through the WAN port of the
    router/gateway. On the other hand, if your network (computer and router) is
    configured to use a subnet mask of then this will be sent
    through the LAN port(s) of the router (not the WAN port).

    Yves Konigshofer, Dec 29, 2004
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.