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 ...

hello

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

In article
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

--
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 ***

On 20 Dec, 01:54, Barry Margolin wrote:
> In article
>
>
>
> 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.

hello

On Dec 20, 1:54 am, Barry Margolin wrote:
> In article
>
>
>
> 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

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

> hello
>
> On Dec 20, 1:54 am, Barry Margolin wrote:
> > In article
> >
> >
> >
> > 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 ***

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

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 ***

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

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

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 )?

Bert

In article
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 ***