Broadcast Subnet??

in CCNA & CCENT
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
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

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127 is the broadcast.
1-126 are valid hosts.
Can you explain why 192.168.7.255 is not the broadcast and and why 127 is?
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
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.
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
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.
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.
thanks
I understand his question, pretty simple for me. However, there's another subnet 192.168.6.0. Why isn't this a broadcast subnet? The broadcast address is 192.168.6.255, which is the broadcast address for a /24 (255.255.255.0) subnet mask.
I don't understand why 192.168.7.128 is a broadcast subnet (with a broadcast address of 192.168.7.255) but 192.168.6.0 isn't a broadcast subnet.
This doesn't make sense. This basically says 192.168.7.128/25 is a broadcast subnet but 192.168.7.0/24 isn't a broadcast subnet. They both have the same broadcast address, so what makes one a broadcast subnet and the other not? The only assumption I can make is that in order for subnets to be classified as a subnet zero (when they're using .0 subnet) is that the subnet has to be using VLSM. Subnets that are sitting on the .0 subnet and not using VLSM cannot be classified as subnet zeros. Correct?
Please help.
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You're almost there, you just haven't figured out the why.
Subnet zero and the broadcast subnet are only considerations when you actually subnet.
192.168.7.0/24 is not a subnetted range, it is a native class C allocation, so if that's what you want, no further subnetting is needed.
192.168.7.128/25 *is* a subnet that falls outside of the classful definitions. The reason it's a broadcast subnet is because it contains the broadcast address (192.168.7.255) for the entire Class C allocation (192.168.7.0/24). This is where the problem used to be - the router didn't know how to differentiate between a broadcast destined for the entire Class C and one destined just for that subnet.
This is why for years the idea was to not use the broadcast subnet for assignment to hosts. That restriction went away awhile ago
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The "zero" subnet, is the subnet number with all subnet bits set to 0.. the "broadcast" subnet has all subnet bits 1'd. It's kind of funny to use the example in this scenario, a /25, because there's only 1 bit for the subnet, giving you two possible subnets (2^1=2.. and they are the 0 and 128 subnets). So the "zero" subnet is 192.168.7.0 /25, and the "broadcast" subnet is 192.168.7.128 /25
It's maybe a little easier to see this when you have multiple subnet bits.. say you used a /28 for that network.. there's 4 bits for subnetting so 2^4=16 subnets with that.. the zero subnet will be 192.168.7.0 /25.. and the broadcast subnet would be the 16th subnet.. 192.168.7.240 /28
If you're subnetting a Class A into multiple /16's or multiple /24's, or a Class B into multiple /24's, there is still going to be a subnet zero. If you stay at a classful boundary and did not subnet, then there will be no subnet zero.
In other words, if you're subnetting, then there will be a subnet zero regardless of what mask you're using.
Can you plz tell me,how come nowadays the router knows how to differentiate between a broadcast destined for the entire Class C and one destined just for that subnet only?
I was curious how this topic was coming up now; I've heard of it, but it's mostly historical. I found the following doc which talks about it in depth:
Subnet Zero and the All-Ones Subnet [IP Addressing Services] - Cisco Systems
The direct answer to your question is: starting with IOS v12.0, Cisco routers will allow the "subnet zero" and the "all ones" subnet by default.
Urvi revived a thread created over 2 years ago, meaning they already did some searching for their answer.
Also they know that both subnet zero and the all ones subnet is allowed. But they don't know how the router is able to differentiate between a broadcast meant for the class broadcast address vs for the broadcast subnet.
I'm pretty sure the broadcast message includes the subnet mask in it. So using the broadcast address, it knows which network it was intended for - whether for the overarching class or the subnet.
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If I had found the answer googling it I wouldn't have posted here. 2ndly,did you find the answer that DoubleNNS said by googling the net?
I didn't!
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