Broadcasting confusion - TCP-IP

This is a discussion on Broadcasting confusion - TCP-IP ; hello I started learning about broadcasting and things are a bit confusing. Hope you can help some. 1) Say we have network N_1 ( 71.94.0.0/15 ) and we divide this network into two subnetworks S_1 ( 71.95.0.0/16 ) and S_2 ...

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Thread: Broadcasting confusion

  1. Broadcasting confusion

    hello

    I started learning about broadcasting and things are a bit confusing.
    Hope you can help some.

    1)
    Say we have network N_1 ( 71.94.0.0/15 ) and we divide this network
    into two subnetworks S_1 ( 71.95.0.0/16 ) and S_2 ( 71.94.0.0/16 ) and
    we further divide S_2 into 4 /18 blocks:

    S_2 - 1 ( 71.95.0.0 /18 )
    S_2 - 2 ( 71.95.64.0 /18 )
    S_2 - 3 ( 71.95.128.0/18 )
    S_2 - 4 ( 71.95.192.0/18 )



    Now say we have computer PC-S_2 ( 71.95.192.1 ) on subnet S_2 - 4 and
    it wants to send:

    a) Broadcast to all subnets of S_2 network.
    Would broadcast IP be 71.95.255.255 ?

    b) Broadcast to all subnets of network N_1 71.95.0.0 ( thus to S_1 and
    S_2 and their subnets ).
    Would broadcast IP also be 71.95.255.255?

    c) In both cases broadcast IP is the same, so how will routers now
    when broadcast is intended for just S_2 subnets and when for both S_1
    and S_2 subnets ?


    2)
    Now say some computer on some network N_2 ( 190.20.20.0/32 ) wants to
    send to network N_1:

    a) Broadcast to all subnets of S_2 network.
    Would broadcast IP be any different than if PC-S_2 would send it?

    b) Broadcast to all subnets of network N_1 ( and thus to S_1 and S_2
    subnets ).
    Again, would broadcast IP be any different than if PC-S_2 would send
    it ?



    3)
    If network is subnetted then I assume net directed broadcast is not
    possible, since the word "net directed" describes ( I assume ) a
    network without any subnetworks?


    thank you

  2. Re: Broadcasting confusion

    In article
    <789f492a-7414-44ac-be21-97a9b003a84b@l32g2000hse.googlegroups.com>,
    failure_to@yahoo.co.uk wrote:

    > hello
    >
    > I started learning about broadcasting and things are a bit confusing.
    > Hope you can help some.
    >
    > 1)
    > Say we have network N_1 ( 71.94.0.0/15 ) and we divide this network
    > into two subnetworks S_1 ( 71.95.0.0/16 ) and S_2 ( 71.94.0.0/16 ) and
    > we further divide S_2 into 4 /18 blocks:
    >
    > S_2 - 1 ( 71.95.0.0 /18 )
    > S_2 - 2 ( 71.95.64.0 /18 )
    > S_2 - 3 ( 71.95.128.0/18 )
    > S_2 - 4 ( 71.95.192.0/18 )
    >
    >
    >
    > Now say we have computer PC-S_2 ( 71.95.192.1 ) on subnet S_2 - 4 and
    > it wants to send:
    >
    > a) Broadcast to all subnets of S_2 network.
    > Would broadcast IP be 71.95.255.255 ?
    >
    > b) Broadcast to all subnets of network N_1 71.95.0.0 ( thus to S_1 and
    > S_2 and their subnets ).
    > Would broadcast IP also be 71.95.255.255?
    >
    > c) In both cases broadcast IP is the same, so how will routers now
    > when broadcast is intended for just S_2 subnets and when for both S_1
    > and S_2 subnets ?


    There's no such thing as "all subnets" broadcasts any more. When CIDR
    was introduced, the distinction between networks and subnets went away.
    You can only send broadcasts to individual subnets, not to multiple
    subnets.

    This is why the all-ones subnet is now usable. Since all-subnets
    broadcasts no longer exist, there's no ambiguity. 71.95.255.255 always
    means broadcast to subnet S_2-4.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  3. Re: Broadcasting confusion

    On 20 Dec, 01:54, Barry Margolin wrote:
    > In article
    > <789f492a-7414-44ac-be21-97a9b003a...@l32g2000hse.googlegroups.com>,
    >
    >
    >
    > failure...@yahoo.co.uk wrote:
    > > hello

    >
    > > I started learning about broadcasting and things are a bit confusing.
    > > Hope you can help some.

    >
    > > 1)
    > > Say we have network N_1 ( 71.94.0.0/15 ) and we divide this network
    > > into two subnetworks S_1 ( 71.95.0.0/16 ) and S_2 ( 71.94.0.0/16 ) and
    > > we further divide S_2 into 4 /18 blocks:

    >
    > > S_2 - 1 ( 71.95.0.0 /18 )
    > > S_2 - 2 ( 71.95.64.0 /18 )
    > > S_2 - 3 ( 71.95.128.0/18 )
    > > S_2 - 4 ( 71.95.192.0/18 )

    >
    > > Now say we have computer PC-S_2 ( 71.95.192.1 ) on subnet S_2 - 4 and
    > > it wants to send:

    >
    > > a) Broadcast to all subnets of S_2 network.
    > > Would broadcast IP be 71.95.255.255 ?

    >
    > > b) Broadcast to all subnets of network N_1 71.95.0.0 ( thus to S_1 and
    > > S_2 and their subnets ).
    > > Would broadcast IP also be 71.95.255.255?

    >
    > > c) In both cases broadcast IP is the same, so how will routers now
    > > when broadcast is intended for just S_2 subnets and when for both S_1
    > > and S_2 subnets ?

    >
    > There's no such thing as "all subnets" broadcasts any more. When CIDR
    > was introduced, the distinction between networks and subnets went away.
    > You can only send broadcasts to individual subnets, not to multiple
    > subnets.
    >
    > This is why the all-ones subnet is now usable. Since all-subnets
    > broadcasts no longer exist, there's no ambiguity. 71.95.255.255 always
    > means broadcast to subnet S_2-4.
    >
    > --
    > Barry Margolin, bar...@alum.mit.edu
    > Arlington, MA
    > *** PLEASE post questions in newsgroups, not directly to me ***
    > *** PLEASE don't copy me on replies, I'll read them in the group ***



    Barry is of course correct.

    I think that you might seek the functionality of Multicast,
    where messages will be forwarded to any client that "subscribes" on a
    specific multicast address ( client uses IGMP messages for this ).

    Any (complete)router will do.

  4. Re: Broadcasting confusion

    hello

    On Dec 20, 1:54 am, Barry Margolin wrote:
    > In article
    > <789f492a-7414-44ac-be21-97a9b003a...@l32g2000hse.googlegroups.com>,
    >
    >
    >
    > failure...@yahoo.co.uk wrote:
    > > hello

    >
    > > I started learning about broadcasting and things are a bit confusing.
    > > Hope you can help some.

    >
    > > 1)
    > > Say we have network N_1 ( 71.94.0.0/15 ) and we divide this network
    > > into two subnetworks S_1 ( 71.95.0.0/16 ) and S_2 ( 71.94.0.0/16 ) and
    > > we further divide S_2 into 4 /18 blocks:

    >
    > > S_2 - 1 ( 71.95.0.0 /18 )
    > > S_2 - 2 ( 71.95.64.0 /18 )
    > > S_2 - 3 ( 71.95.128.0/18 )
    > > S_2 - 4 ( 71.95.192.0/18 )

    >
    > > Now say we have computer PC-S_2 ( 71.95.192.1 ) on subnet S_2 - 4 and
    > > it wants to send:

    >
    > > a) Broadcast to all subnets of S_2 network.
    > > Would broadcast IP be 71.95.255.255 ?

    >
    > > b) Broadcast to all subnets of network N_1 71.95.0.0 ( thus to S_1 and
    > > S_2 and their subnets ).
    > > Would broadcast IP also be 71.95.255.255?

    >
    > > c) In both cases broadcast IP is the same, so how will routers now
    > > when broadcast is intended for just S_2 subnets and when for both S_1
    > > and S_2 subnets ?

    >
    > There's no such thing as "all subnets" broadcasts any more. When CIDR
    > was introduced, the distinction between networks and subnets went away.
    > You can only send broadcasts to individual subnets, not to multiple
    > subnets.
    >


    Uh, so we can't even do all S_2-subnets broadcast ?!


    When you divide N_1 into S_1 and S_2 and S_2 into 4 additional subnets
    --> is this done all on one router? Such that this router has
    altogether at least 5 NICs ( one for S_1, one for S_2-1, one for
    S_2-2, one for S_2-3 and one for S_2-4 )?

    thank you

  5. Re: Broadcasting confusion

    In article
    ,
    failure_to@yahoo.co.uk wrote:

    > hello
    >
    > On Dec 20, 1:54 am, Barry Margolin wrote:
    > > In article
    > > <789f492a-7414-44ac-be21-97a9b003a...@l32g2000hse.googlegroups.com>,
    > >
    > >
    > >
    > > failure...@yahoo.co.uk wrote:
    > > > hello

    > >
    > > > I started learning about broadcasting and things are a bit confusing.
    > > > Hope you can help some.

    > >
    > > > 1)
    > > > Say we have network N_1 ( 71.94.0.0/15 ) and we divide this network
    > > > into two subnetworks S_1 ( 71.95.0.0/16 ) and S_2 ( 71.94.0.0/16 ) and
    > > > we further divide S_2 into 4 /18 blocks:

    > >
    > > > S_2 - 1 ( 71.95.0.0 /18 )
    > > > S_2 - 2 ( 71.95.64.0 /18 )
    > > > S_2 - 3 ( 71.95.128.0/18 )
    > > > S_2 - 4 ( 71.95.192.0/18 )

    > >
    > > > Now say we have computer PC-S_2 ( 71.95.192.1 ) on subnet S_2 - 4 and
    > > > it wants to send:

    > >
    > > > a) Broadcast to all subnets of S_2 network.
    > > > Would broadcast IP be 71.95.255.255 ?

    > >
    > > > b) Broadcast to all subnets of network N_1 71.95.0.0 ( thus to S_1 and
    > > > S_2 and their subnets ).
    > > > Would broadcast IP also be 71.95.255.255?

    > >
    > > > c) In both cases broadcast IP is the same, so how will routers now
    > > > when broadcast is intended for just S_2 subnets and when for both S_1
    > > > and S_2 subnets ?

    > >
    > > There's no such thing as "all subnets" broadcasts any more. When CIDR
    > > was introduced, the distinction between networks and subnets went away.
    > > You can only send broadcasts to individual subnets, not to multiple
    > > subnets.
    > >

    >
    > Uh, so we can't even do all S_2-subnets broadcast ?!


    Correct. Broadcasts only exist on leaf subnets, not aggregates.

    > When you divide N_1 into S_1 and S_2 and S_2 into 4 additional subnets
    > --> is this done all on one router? Such that this router has
    > altogether at least 5 NICs ( one for S_1, one for S_2-1, one for
    > S_2-2, one for S_2-3 and one for S_2-4 )?


    It could be done that way, but it doesn't have to be. Some of the
    subnets could be at remote offices, others in the main headquarters.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  6. Re: Broadcasting confusion

    hello


    On Dec 21, 4:53 am, Barry Margolin wrote:
    > In article
    > ,
    >
    >
    >
    > failure...@yahoo.co.uk wrote:
    > > hello

    >
    > > When you divide N_1 into S_1 and S_2 and S_2 into 4 additional subnets
    > > --> is this done all on one router? Such that this router has
    > > altogether at least 5 NICs ( one for S_1, one for S_2-1, one for
    > > S_2-2, one for S_2-3 and one for S_2-4 )?

    >
    > It could be done that way, but it doesn't have to be. Some of the
    > subnets could be at remote offices, others in the main headquarters.
    >


    That is what I'm having hard time understanding. If router R1 has 4
    NICs ( one for S_2-1, one for S_2-2, one for S_2-3 and one for
    S_2-4 ) but some other router R2 ( which is on same LAN as R1 ) has
    S-1 NIC, then what is address of network lying between these two
    routers? I'm assuming it can't be N1, since N1 is already subnetted
    to S_1 and S_2?!


    cheers mate



  7. Re: Broadcasting confusion

    In article
    ,
    failure_to@yahoo.co.uk wrote:

    > hello
    >
    >
    > On Dec 21, 4:53 am, Barry Margolin wrote:
    > > In article
    > > ,
    > >
    > >
    > >
    > > failure...@yahoo.co.uk wrote:
    > > > hello

    > >
    > > > When you divide N_1 into S_1 and S_2 and S_2 into 4 additional subnets
    > > > --> is this done all on one router? Such that this router has
    > > > altogether at least 5 NICs ( one for S_1, one for S_2-1, one for
    > > > S_2-2, one for S_2-3 and one for S_2-4 )?

    > >
    > > It could be done that way, but it doesn't have to be. Some of the
    > > subnets could be at remote offices, others in the main headquarters.
    > >

    >
    > That is what I'm having hard time understanding. If router R1 has 4
    > NICs ( one for S_2-1, one for S_2-2, one for S_2-3 and one for
    > S_2-4 ) but some other router R2 ( which is on same LAN as R1 ) has
    > S-1 NIC, then what is address of network lying between these two
    > routers? I'm assuming it can't be N1, since N1 is already subnetted
    > to S_1 and S_2?!


    I'm a little confused by your example. R2 should have at least TWO
    NICs, otherwise it's not routing between anything. So it can have a NIC
    on S_2-1, and this can be used as the address of the network between the
    two routers.

    You also could use a totally unrelated network for the network between
    the two routers.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  8. Re: Broadcasting confusion

    On Dec 23, 9:17*am, failure...@yahoo.co.uk wrote:

    > > > When you divide N_1 into S_1 and S_2 and S_2 into 4 additional subnets
    > > > --> is this done all on one router? Such that this router has
    > > > altogether at least 5 NICs ( one for S_1, one for S_2-1, one for
    > > > S_2-2, one for S_2-3 and one for S_2-4 )?

    >
    > > It could be done that way, but it doesn't have to be. *Some of the
    > > subnets could be at remote offices, others in the main headquarters.

    >
    > That is what I'm having hard time understanding. If router R1 has 4
    > NICs ( one for S_2-1, one for *S_2-2, one for S_2-3 and one for
    > S_2-4 ) but some other router R2 ( which is on same LAN as R1 ) has
    > S-1 NIC, then what is address of network lying between these two
    > routers? *I'm assuming it can't be N1, since N1 is already subnetted
    > to S_1 and S_2?!


    It can be any other net.

    There are a few fundamental concepts underlying your questions, I
    think. One of them is that "broadcast" with IPv4 is really limited now
    to just broadcasting within a single subnet. In IPv6, even that's not
    done. Neighbor Discovery (ND, the equivalent of ARP for IPv6) and
    routing protocols, two examples of heavy use of broadcast in IPv4, use
    multicast rather than broadcast, in IPv6.

    The other concept is that geographically adjacent networks do not
    necessarily have to have related IP prefixes. The way you subnetted
    your IP nets is convenient for routers. If the entire Internet were
    organized this way, you would have a relatively small number of routes
    to define at core routers. It's called "address aggregation." Useful
    as it is, it's not mandatory.

    A related point is that for all the talk about how IPv6 allows for
    kozillions of individual IP addresses, which it does, the routing
    problem is really not alleviated by IPv6. Which is to say, if these
    kozillion IPv6 hosts are connected to some enormous number of
    different and non-aggregatable (?) IPv6 subnets, there's nothing
    inherent in IPv6 that makes it easier for core routers to store that
    hugely increased number IPv6 routes.

    Bert

  9. Re: Broadcasting confusion

    hello

    On Dec 23, 7:27 pm, Barry Margolin wrote:
    > In article
    > ,
    >
    >
    >
    > failure...@yahoo.co.uk wrote:
    > > hello

    >
    > > On Dec 21, 4:53 am, Barry Margolin wrote:
    > > > In article
    > > > ,

    >
    > > > failure...@yahoo.co.uk wrote:
    > > > > hello

    >
    > > > > When you divide N_1 into S_1 and S_2 and S_2 into 4 additional subnets
    > > > > --> is this done all on one router? Such that this router has
    > > > > altogether at least 5 NICs ( one for S_1, one for S_2-1, one for
    > > > > S_2-2, one for S_2-3 and one for S_2-4 )?

    >
    > > > It could be done that way, but it doesn't have to be. Some of the
    > > > subnets could be at remote offices, others in the main headquarters.

    >
    > > That is what I'm having hard time understanding. If router R1 has 4
    > > NICs ( one for S_2-1, one for S_2-2, one for S_2-3 and one for
    > > S_2-4 ) but some other router R2 ( which is on same LAN as R1 ) has
    > > S-1 NIC, then what is address of network lying between these two
    > > routers? I'm assuming it can't be N1, since N1 is already subnetted
    > > to S_1 and S_2?!

    >
    > I'm a little confused by your example. R2 should have at least TWO
    > NICs, otherwise it's not routing between anything. So it can have a NIC
    > on S_2-1, and this can be used as the address of the network between the
    > two routers.
    >

    I'm sorry about that. I'm not very good at conveying my thoughts, so I
    will give it another try:

    * R1 has four N1 NICs ( one for S_2-1, one for S_2-2, one for S_2-3
    and one for
    S_2-4 )
    * One of R2's NICs is on subnetwork S_1
    * R1 is connected to R2 with NIC called R1_not_on_S_1 ( R2 is
    connected to R1 with R2_not_on_S_1 )
    * R1_not_on_S_1 is not on S_2-1 or S_2-2 or S_2-3 or S_2-4 or S_1
    subnetwork, so I'm guessing that network between R1 and R2 can't be
    N1, but instead has to be some other, unrelated network?


    > The other concept is that geographically adjacent networks do not
    > necessarily have to have related IP prefixes. The way you subnetted
    > your IP nets is convenient for routers. If the entire Internet were
    > organized this way, you would have a relatively small number of routes
    > to define at core routers. It's called "address aggregation." Useful
    > as it is, it's not mandatory.


    If subnets of N1 ( S_1 and S_2 ) are more or less directly connected,
    then outside routers can have just one N1 table entry
    ( 71.94.0.0/15 ), but if S_1 and S_2 are, say on different sides of
    the world, then outside routers need two entries in their routing
    tables ( one specifying S_1 and other specifying S_2 )?


    thank you

  10. Re: Broadcasting confusion

    On Dec 25, 8:10*pm, failure...@yahoo.co.uk wrote:
    >
    > > The other concept is that geographically adjacent networks do not
    > > necessarily have to have related IP prefixes. The way you subnetted
    > > your IP nets is convenient for routers. If the entire Internet were
    > > organized this way, you would have a relatively small number of routes
    > > to define at core routers. It's called "address aggregation." Useful
    > > as it is, it's not mandatory.

    >
    > If subnets of N1 ( S_1 and S_2 ) are more or less directly connected,
    > then outside routers can have just one N1 table entry
    > ( 71.94.0.0/15 ), but if S_1 and S_2 are, say on different sides of
    > the world, then outside routers need two entries in their routing
    > tables ( one specifying S_1 and other specifying S_2 )?


    Yes. That's the nice thing about address aggregation.

    Bert

  11. Re: Broadcasting confusion

    In article
    <12e1b372-8bd7-4096-a80f-9425e3fdb8df@p69g2000hsa.googlegroups.com>,
    failure_to@yahoo.co.uk wrote:

    > hello
    >
    > On Dec 23, 7:27 pm, Barry Margolin wrote:
    > > In article
    > > ,
    > >
    > >
    > >
    > > failure...@yahoo.co.uk wrote:
    > > > hello

    > >
    > > > On Dec 21, 4:53 am, Barry Margolin wrote:
    > > > > In article
    > > > > ,

    > >
    > > > > failure...@yahoo.co.uk wrote:
    > > > > > hello

    > >
    > > > > > When you divide N_1 into S_1 and S_2 and S_2 into 4 additional subnets
    > > > > > --> is this done all on one router? Such that this router has
    > > > > > altogether at least 5 NICs ( one for S_1, one for S_2-1, one for
    > > > > > S_2-2, one for S_2-3 and one for S_2-4 )?

    > >
    > > > > It could be done that way, but it doesn't have to be. Some of the
    > > > > subnets could be at remote offices, others in the main headquarters.

    > >
    > > > That is what I'm having hard time understanding. If router R1 has 4
    > > > NICs ( one for S_2-1, one for S_2-2, one for S_2-3 and one for
    > > > S_2-4 ) but some other router R2 ( which is on same LAN as R1 ) has
    > > > S-1 NIC, then what is address of network lying between these two
    > > > routers? I'm assuming it can't be N1, since N1 is already subnetted
    > > > to S_1 and S_2?!

    > >
    > > I'm a little confused by your example. R2 should have at least TWO
    > > NICs, otherwise it's not routing between anything. So it can have a NIC
    > > on S_2-1, and this can be used as the address of the network between the
    > > two routers.
    > >

    > I'm sorry about that. I'm not very good at conveying my thoughts, so I
    > will give it another try:
    >
    > * R1 has four N1 NICs ( one for S_2-1, one for S_2-2, one for S_2-3
    > and one for
    > S_2-4 )
    > * One of R2's NICs is on subnetwork S_1
    > * R1 is connected to R2 with NIC called R1_not_on_S_1 ( R2 is
    > connected to R1 with R2_not_on_S_1 )
    > * R1_not_on_S_1 is not on S_2-1 or S_2-2 or S_2-3 or S_2-4 or S_1
    > subnetwork, so I'm guessing that network between R1 and R2 can't be
    > N1, but instead has to be some other, unrelated network?


    Right. Or you could divide one of your networks further, and use that
    for the subnet connecting the two routers.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

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