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subnet all 0 or all 1
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| i don't get why the subnets 192.65.120.0/28 and 192.65.120.240/28 cannot be considered as normal subnets using the mask 255.255.255.240
on the subnet 0, the range for ip-host address would be:
from 192.65.120.1 to 192.65.120.238
the broadcast address on this subnet would be 192.65.120.239
on the subnet 240, the range for ip-host address would be:
from 192.65.120.241 to 192.65.120.254
the broadcast address on this subnet would be 192.65.120.255
The host ip-address rule specify that the host-address cannot be all 1 or all 0. correct?
if we extend this rule to the network-address, we could say that the network-address cannot be all 1 or all 0. correct?
my question is:
why should we exclude the subnets
192.65.120.0/28 and 192.65.120.240/28 considering that the rule is still respected?
thank for your help.
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| heres your answer
network hosts broadcast
.0 .01-.14 .15
.16 .17-.30 .31
.32 .33-.46 .47
.48 .49-.62 .63
.64 .65-.78 .79
.80 .81-.94 .95
.96 .97-.110 .111
.112 .113-.126 .127
.128 .129-.142 .143
.144 .145-.158 .159
.160 .161-.174 .175
.176 .177-.190 .191
.192 .193-.206 .207
.208 .209-.222 .223
.224 .225-.238 .239
.240 .241-.254 .255
now by rule(unless ip subnet zero is enabled)you drop the first and last subnets. so here we have a total of 16 subnets -2= 14 usable subnets(unless ip subnet zero is enabled) | |
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| heres your answer
network --- hosts --- broadcast
.0 --------.01-.14--------.15
.16--------.17-.30--------.31
.32--------.33-.46--------.47
.48--------.49-.62--------.63
.64--------.65-.78--------.79
.80--------.81-.94--------.95
.96--------.97-.110------.111
.112------.113-.126------.127
.128------.129-.142------.143
.144------.145-.158------.159
.160------.161-.174------.175
.176------.177-.190------.191
.192------.193-.206------.207
.208------.209-.222------.223
.224------.225-.238------.239
.240------.241-.254------.255
now by rule(unless ip subnet zero is enabled)you drop the first and last subnets. so here we have a total of 16 subnets -2= 14 usable subnets(unless ip subnet zero is enabled) | |
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| i don't understand why these hosts are not valid:
from 192.65.120.1/28 to 192.65.120.238/28
from 192.65.120.241/28 to 192.65.120.255/28
could you send me to a site where it would be explained/
thanks a lot
frans | |
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| Hey there,
Let me take a crack at your problem. when ever you get an ip you must try to determin the Network ID and the Host ID. Try to convert them into binary first it will make it easier.
1) 192.65.120.0
255.255.255.240
1100 0000.0100 0001.0111 1000.0000 0000
1111 1111.1111 1111.1111 1111.1111 0000
This is your Networ Portion:
1100 0000.0100 0001.0111 1000.0000
1111 1111.1111 1111.1111 1111.1111
This is your Host portion:
0000
0000
you have to remmember that all bits above 1's (subnet mask) are your network id. and all bits above zero's are host id .So in our 1st senario, your network Id is 192.65.120.0 and your host id is X.X.X.0. Now as you know that all bits can't be all 1's or 0's in network or host id. thats why this is an invalid ip add.
i hope this helps and try converting the 2nd one your self and will see why that one is invalid aswell. Try to get your hands on Tod Lammles CCNA study guide by Sybex. He makes subnetting very simple to understand.
Goodluck!!
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traveller
MCP+I, MCSE, CCNA2.0
[This message has been edited by traveller (edited 09-27-2000).] | |
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| this link from Cisco's site explains the subnet zero and all 1's.
http://www.cisco.com/warp/public/105/40.html
the rule is for the subnet, not the entire network portion. So for instance the class B address 172.16.0.129/25 doesn't violate this rule, but the address 172.16.0.127/25 as well as 172.16.255.129/25 does.
try the addresses on any pre 12.0 IOS until you understand it. the router will let you know if it's invalid. Then enable subnet-zero and try again. with 12.0, subnet-zero is now enabled by default.
[This message has been edited by Detour (edited 09-27-2000).] | |
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| Here's what I think:
(A) The Cisco IOS will always see an IP address like 192.65.120.0 as one network, including ALL subnets. (unless ip subnet zero is enabled).
(B) It sees 192.65.120.255 as a broadcast address for network 192.65.120.0. That means every nodes in ALL subnets will receive the broadcasted packets.
Subnet 192.65.120.0 is excluded because of (A) above.
Subnet 192.65.120.240 is excluded because of
(B) above.
[This message has been edited by hwl130 (edited 09-27-2000).] | |
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| From my understanding of subnets, the subnet addresses
1) 192.65.120.0/28
2) 192.65.120.240/28
with mask 255.255.255.240 contain:
1) Host addresses 192.65.120.1 to
192.65.120.14, broadcast address 192.65.120.15
2) Host addresses 192.65.120.241 to
192.65.120.254, broadcast address
192.65.120.255
In response to your question, there seems to be no reason why each of the subnets cannot be treated as normal subnets. HOWEVER, I note that when you specify subnet 0 in your original message, you include ALL addresses between 192.65.120.1 to 192.65.120.238 as host addresses. Using the above subnet mask, you are allowed only 16 addresses per subnet, one for subnet address and the other for broadcast address. By specifying all addresses in the above range for subnet 0, you take up addresses for other subnets.
Whenever I get any of these probs, I clearly write out the following:
1) Host address (if provided)
2) Subnet mask
3) Subnet address
4) Broadcast address
5) First address in subnet
6) Last address in subnet
Using this method has never failed me (to date)
Hope this helps,
Deets | |
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| Actually, I'm going to do a slight about-face on the last post (sounds very military!) The two subnets you specify are the first and last subnets available within that ip grouping. According to Cisco, these are reserved and should not be used. For more interesting debate about this topic, refer to DJDZME5's post.
Cheers,
Deets | |
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| thanks for your comments.
supposing a different scenario.
i have 4 floors, and i want 1 subnet/each floor.
I suggest these networks:
10.1.2.0/26
10.1.2.64/26
10.1.2.128/26
10.1.2.192/26
do you agree?
frans | |
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| hey there,
what wrong with
1) 10.1.10.X 255.255.255.0
2) 10.1.20.X 255.255.255.0
3) 10.1.30.X 255.255.255.0
4) 10.1.40.X 255.255.255.0
This way for each subnet you get to assign upto 254 host. X represents the host add.This ip allows you 65,534 valid subnets and 254 host for each subnet.
Regards
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traveller
MCP+I, MCSE, CCNA2.0 | |
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| hey there again,
Or you could have
1) 10.10.X.X 255.255.0.0
2) 10.20.X.X 255.255.0.0
3) 10.30.X.X 255.255.0.0
3) 10.40.X.X 255.255.0.0
This senario gives you 254 valid subnets and you could assign 65,534 host per subnet. Again X represents host add's. | |
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| thanks traveller
but i wish you checked the exact addresses and subnet masks that i gave in my scenario.
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| This would not accomodate the number of subnets needed due to the formula 2^n-2. This, of course, is if you are not using the subnet zero as has been mentioned. I would only use subnet zero if I was trying to save myself a few subnets, and then restrict the reserved subnets for links between routers.
Reamer
[This message has been edited by The Reamer (edited 09-29-2000).] | |
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| Sorry to ruin the clarity guys but in the Cisco Press Book Exam#640-507 Certification
Guide on pages 265 and 266 subnets is computed with 2!(subnet bits). The author even mentions in a note that 'In some documents,the number of subnets is shown as two less than 2!(number of subnet bits),implying two reserved cases.These cases are no longer reserved,however, and can be used.' What the exam uses in computing the number of subnets:2!n or 2!n-2 ? Search me! | |
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| Don't be sorry, DanK. I was going to voice the same concern at some stage. Simple and straight-forward:
Question 1: On the EXAM, do we treat the number of subnets as 2^n - 2, or 2^n, if they DON'T mention anything about subnet zero.
Question 2: On the EXAM, if we get a question such as: What are the first and last available subnets given network number 150.150.0.0 and mask 255.255.248.0?
In the second one, you could have either of the following answers:
150.150.0.0 and 150.150.248.0 (this would violate the subnet zero rule and subnet one rule)
or
150.150.8.0 and 150.150.240.0.
Please! Some concrete answers in relation to what the EXAM wants! I understand this topic like the back of my hand under a spotlight, yet due to technicalities of wording may still get it wrong on the exam.
Deets  |
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