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| justindu 2003-09-26, 10:29 am |
| Which of the following addresses do not share the same subnet number?
IP = 151.100.225.1; Mask = 255.255.192.0
IP = 151.100.193.100; Mask =255.255.192.0
IP = 151.100.156.1; Mask = 255.255.192.0
IP = 151.100.255.210; Mask = 255.255.192.0
Why is the answer:
IP = 151.100.156.1; Mask = 255.255.192.0 | |
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| Well a mask of 255.255.192.0 will basically split your class B subnet in half - i.e, the first half will consist of 151.100.0.0 - 151.100.191.255, the second half will be 151.100.192.0 - 151.100.255.255.
So the first two and the last one all fall into the same subnet range when that mask is used and 151.100.156.1 will be in another subnet.
Make sense??  | |
| justindu 2003-09-26, 10:49 am |
| Sort of..
Why are the subnets not incremented by 64 though? | |
| NetStroller 2003-09-26, 11:09 am |
| Actually the subnets do increment by 64, and all but the third address is in the 151.100.192.0 network. Valid hosts 151.100.192.1 through 151.100.255.254. Feel free to correct me, just doing this from my head and I'm not a net admin. | |
| justindu 2003-09-26, 11:57 am |
| Ok I got it..
So the thrid address would actually be in the 151.100.128.0 net right? | |
| asteheske 2003-09-26, 2:21 pm |
| Class B : 150.100.0.0 network with the subnet mask 255.255.192.0
1st Ip
Network : 150.100.0.0
Host : 150.100.0.1 to 150.100.63.254
Brdcast : 150.100.63.255
2nd Ip
Network : 150.100.64.0
Host : 150.100.64.1 to 150.100.127.254
Brdcast : 150.100.127.255
3rd Ip
Network : 150.100.128.0
Host : 150.100.128.1 to 150.100.191.254
Brdcast : 150.100.191.255
4th Ip
Network : 150.100.192.0
Host : 150.100.192.1 to 150.100.255.254
Brdcast : 150.100.255.255
As you can see the :
IP = 151.100.225.1; Mask = 255.255.192.0
IP = 151.100.193.100; Mask =255.255.192.0
IP = 151.100.255.210; Mask = 255.255.192.0
all belongs to the 4th Ip table
And the :
IP = 151.100.156.1; Mask = 255.255.192.0
is the only one that belongs to the 3rd Ip table
Hope this helps  ~~~ | |
| lollivie 2003-09-26, 5:46 pm |
| I am trying to understand your reply about the 4 subnets.
I was wondering if there should only be 2 valid subnets. 192 is 2 bits of subnetting and 2 * 2 = 4 -2 results in 2 subnets.
Then only the middle two subnets would be valid to use.
Is this correct?
Larry | |
| Tech Ranger 2003-09-26, 10:49 pm |
| quote:
Originally posted by lollivie
I am trying to understand your reply about the 4 subnets.
I was wondering if there should only be 2 valid subnets. 192 is 2 bits of subnetting and 2 * 2 = 4 -2 results in 2 subnets.
Then only the middle two subnets would be valid to use.
Is this correct?
Larry
I was thinking the very same thing. What happened to all ones and all zeros are no good? | |
| Yankee 2003-09-27, 7:50 am |
| Forget the Microsoft teaching and join the real world! asteheske posted the correct answer
Yankee | |
| asteheske 2003-09-27, 4:26 pm |
| If a network address is subnetted, the first subnet obtained after subnetting the network address is called subnet zero . And the last subnet obtained is called the all-ones subnet
Problems with Subnet Zero and the All-Ones Subnet:
IT was strongly recommended that subnet zero and the all-ones subnet not be used for addressing. According to rfc 950 , "It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets." This is the reason why network engineers required to calculate the number of subnets obtained by borrowing three bits would calculate 23-2 (6) and not 23 (8). The "-2" takes into account that subnet zero and the all-ones subnet are not used traditionally.
Using Subnet Zero and the All-Ones Subnet:
It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet has always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS version 12.0. Even prior to Cisco IOS version 12.0, subnet zero could be used by entering the ip subnet-zero global configuration command.
Using Subnet Zero and the All-Ones Subnet:
It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet has always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS version 12.0. Even prior to Cisco IOS version 12.0, subnet zero could be used by entering the ip subnet-zero global configuration command.
On the issue of using subnet zero and the all-ones subnet, rfc 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete! Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use, though, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.
These are the explaination frm cisco with regards to subnetting you can read more
here
And to add that, the question asked by justindu:
quote:
Which of the following addresses do not share the same subnet number?
I created the subnet table to give a clearer picture of how they get the answer.
I hope this will clear the misundertanding and help you with the subnetting (ie: Cisco Style)
Cheers~~ |
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