why not use broadcast subnet.

[sarahr202]

Hi everybody

I am just wondering why don't we use broadcast subnet when we can use it. For example

we have a subnet  172.16.240.0/255.255.240.0

R1----f0/0------------------------------f0/0--R2

R1 f0/0

ip address 172.16.240.1/20

R2 f0/0

ip address 172.16.240.2/20

I am able to ping 172.16.240.1 from R1.

My question since we can use broadcast subnet then why don't we use it ?

thanks and have a great day.

[John Blakley · Accepted solution]

Sarah,

The broadcast address for your example above is 172.16.255.255. The broadcast is a special address that sends a packet to all hosts in the same subnet. You can't assign a broadcast address to an interface in a router:

Router(config-if)#ip address 192.168.1.255 255.255.255.0

Bad mask /24 for address 192.168.1.255

Router(config-if)#

Here's the RFC for it:

http://tools.ietf.org/html/rfc919

HTH,
John

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[Reza Sharifi · Accepted solution]

Hi Sarah,

You mean broadcast address not subnet right?

Each subnet has a broadcast address.

HTH

[Giuseppe Larosa · Accepted solution]

Hello Sarah,

since 12.0 IOS the default behaviour of routers is  ip classless routing ( ip classless) and this allows for use of the first and last IP subnets as you have successfully verified.

This behaviour refers to the use of IP routing table and it is not the same as classless routing protocols (all the modern ones RIPV2, EIGRP, OSPF, ISIS and BGP).

To make another important example of the classless behaviour of routers:

in classless routing a default route is used also for unknown IP subnets of connected major networks ( class A,B, C network)

in classful routing the default route is not used for unknown IP subnets of a connected major network and a packet with a destination belonging to an unknown IP subnet of a connected major network is silently discarded

So the old subnetting rule was: when moving to the right of N bits the subnet boundary, given a subnetting that would produce 2^N IP subnets the usable subnets are 2^N-2 (not usable the first IP subnet with subnet mask = to major network subnet mask and not usable last IP subnet for the broadcast overlapping).

The modern IP subnetting rule says: if you move of N bits the subnet boundary to the right you can use up to 2^N IP subnets including the first and the last one.

However, when first training books have been written the classful behaviour was the default, and for some reasons still some books at CCNA level or above report the old subnetting rule that has no reasons to be used.

I would explain the modern subnetting rule directly.

I remember when I have attended the CCNP 1.0 Routing course we had been requested in a lab to activate this ip classless feature but routers were already in 12.0 ( june 2000 12 years ago!).

This is an example of a legacy concept that has been reported in newer editions of books.

Hope to help

Giuseppe

[cadet alain · Accepted solution]

Hi Sarah,

are you talking about the broadcast subnet and zero subnet that were not useable in the prehistory of networking?

In which case your example is not a broadcast subnet such a subnet is one that has broadcast value same as the classful network broadcast value.

for example :

192.168.1.0/30 is a zero subnet because the subnet address is same as 192.168.1.0/24

192.168.1.192/26 is a broadcast subnet because the broadcast address is 192.168.1.255 same as 192.168.1.0/24

But now they are useable and used in networks.

Regards.

Alain

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[sarahr202]

Thanks Alain.

Are you studying for ccie?

Happy Holidays!

[cadet alain]

Hi Sarah,

Yes I'm studying for CCIE RS.

Regards.

Alain

Don't forget to rate helpful posts.

[sarahr202]

Good luck with your study.  You are one of those people on this forum who really inspires me .

Happy Holidays!

[cadet alain]

Hi Sarah,

thanks, happy holiday to you too.

Regards.

Alain

Don't forget to rate helpful posts.

[Peter Paluch · Accepted solution]

Hi Sarah,

In addition to all nice explanations posted here, you may be interested to read this document:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

Best regards,

Peter

[sarahr202]

Sorry for the late reply

No , I am talking about broadcast subnet.  I understand the concept of broadcast address

But I got it.It was the old concept .

Let me read the links you guys posted here. Now I am trying to find the reason why we did not use broadcast subnet in old subnet rules.

thanks

[Joseph W. Doherty · Accepted solution]

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Posting

When you're looking at the other postings and their provided references, keep in mind the key conceptual difference between classful and classless routing and what "subnets" really mean in classful routing.  The term "subnets", especially today, gets thrown about without the full meaning it had with classful addressing.

In answer to you question "My question since we can use broadcast subnet then why don't we use it?", often it's because this is a pratice that's been "passed down" and many engineers are used to NOT using it even when they could.