Broadcast Subnet??

[Andrew Newman]

1) I'm looking at a Boson practice test for ICND1 where it states that a subnet with the subnet number 192.168.7.128 and mask 255.255.255.128 is a broadcast subnet. Is this because there are only 1's in the 4th octet?

2) Along the same lines is the subnet number 172.30.0.0 /21 considered to be a ZERO subnet because there are only 0's in the 3rd octet?

thanks for any help

[knwminus]

Originally Posted by Andrew Newman

1) I'm looking at a Boson practice test for ICND1 where it states that a subnet with the subnet number 192.168.7.128 and mask 255.255.255.128 is a broadcast subnet. Is this because there are only 1's in the 4th octet?

2) Along the same lines is the subnet number 172.30.0.0 /21 considered to be a ZERO subnet because there are only 0's in the 3rd octet?

thanks for any help

127 is the broadcast.

1-126 are valid hosts.

[Andrew Newman]

Hi, thanks for that.
Can you explain why 192.168.7.255 is not the broadcast and and why 127 is?

[Forsaken_GA]

There's a little bit of confusion.

255.255.255.128 is a /25, so you get two subnets out of that, 128 ip's each

the first subnet is 192.168.7.0 to 192.168.7.127

0 is the network identifier, 127 is the broadcast IP

192.168.7.128 to 192.168.7.255 is the second subnet

128 is the network identifier, 255 is the broadcast

so Boson is correct, 192.168.7.128 is a broadcast subnet, since it contains the broadcast IP for the /24 that was split up

[knwminus]

Originally Posted by Andrew Newman

Hi, thanks for that.
Can you explain why 192.168.7.255 is not the broadcast and and why 127 is?

Yes. The broadcast for the subnet is the last number before the next number. A /25 means 2 networks in the 4th octet.

0-126


127 is broadcast.

What is the broadcast for the network?

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@For

Ok I understand what you are saying. Forsaken is right and he explanation makes sense.

[Andrew Newman]

Hi, I'm getting closer to understanding it now.I can see why /25 for a class C address means 2 possible subnets but I still don't see why 192.168.7.128 is a broadcast subnet and not just a regular subnet.

I don't get what you mean when you say 'so Boson is correct, 192.168.7.128 is a broadcast subnet, since it contains the broadcast IP for the /24 that was split up'
Can you expand on that a bit please, some zeros and noughts would be good .I'm new to this and it's hurting my head something chronic but i'll keep at it until I've got it.

thanks

[Forsaken_GA]

Originally Posted by Andrew Newman

Hi, I'm getting closer to understanding it now.I can see why /25 for a class C address means 2 possible subnets but I still don't see why 192.168.7.128 is a broadcast subnet and not just a regular subnet.

I don't get what you mean when you say 'so Boson is correct, 192.168.7.128 is a broadcast subnet, since it contains the broadcast IP for the /24 that was split up'
Can you expand on that a bit please, some zeros and noughts would be good .I'm new to this and it's hurting my head something chronic but i'll keep at it until I've got it.

thanks

Because if it wasn't subnetted, that would normally be a /24, as 192.168 is, in classful terms, a class C

For that /24, the range would be 192.168.7.0 to 192.168.7.255, with .0 being the network identifier, and .255 being the broadcast.

If it wasn't subnetted, 192.168.7.255 would be the broadcast IP for the network.

Since it is subnetted and broken into two networks, 192.168.7.128/25 contains the broadcast IP of the classful allocation, hence it's a broadcast subnet

[Forsaken_GA]

You also need to understand that the concept is pretty obsolete, but the CCNA wants you to learn it. When I went through the CCNA net academy, we were taught that when you're busting up a classful network allocation into subnets, you can't use the first subnet (the all zeros) or the last subnet (the all ones) because the first subnet contains the network identifier for the entire network and the last subnet contains the broadcast for the entire network.

This was true when subnetting was first introduced, the equipment and the IP stacks didn't really know how to handle that (were you trying to send a broadcast to the entire network, or just the last subnet!) Today, it doesn't matter, the all-zeros subnet and the all-ones subnet are perfectly useable, since the classful method of operation has fallen by the wayside. It's possible that it could become an issue if you have legacy equipment and implementations in your network, however, which is why I guess they want you to learn about it.

[tha_dub]

I think foresaken has hit the nail on the head.

The Broadcast subnet is the highest subnet. so for 192.168.7.128 /25 you only have 2 subnets 192.168.7.0 and 192.168.7.128. the first is the zero subnet and the last (highest subnet) is the broadcast subnet. The only time you would really pay much attention to this is if the router had the "no ip subnet zero" command in which case both subnets would be unusable.

Assuming you have No ip subnet zero with say 192.168.1.1/26.

you'd have
192.168.1.0 (Zero subnet)
192.168.1.64 usable
192.168.1.128 usable
192.168.1.192 (broadcast subnet)

Basically you need to know this info for the test in case they tell you they want to know the broadcast or zero subnet or if they router has a no ip subnet zero command. Otherwise assume you can use the zero and broadcast subnets.

[Andrew Newman]

I get it now.
thanks