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| JayDot 2002-02-21, 7:47 am |
| Ok I Thought When Given a
Class A IP You should Use a Class A
255.x.x.x Subnet Mask
Class B IP
255.255.x.x Class B Subnet Mask
Class C IP
255.255.255.x Class C Subnet Mask
I ran across a TechSkills Test Prep Question
That used a Class B Subnet Mask for Class A IP Address.... Can u Use any Class Subnet
and Why Would I Want to Do This ???
Test Question:
What is The Network Number for the IP
10.128.100.224 with subnet mask 255.255.240.0
1)10.128.96.0
2)10.128.92.0
3)10.128.0.0
4)10.128.255.255
The Answer was #1 | |
| Hippo 2002-02-21, 10:44 am |
| JayDot
there is no reason at all why you should not use a class B mask on a class A network. In fact, why not use a class C mask on a class B or a class A network.
The DEFAULT mask for a class A network is 255.0.0.0 and for a class B is 255.255.0.0.
In your test question, the use of a class B mask on a class A network simply allows the host portion of the class A address (in the second, third and fourth octet) to be used for additional networks, or more properly, more subnets from the class A address. This is not uncommon; in fact when you get to CCNP and start looking at Classless Inter Domain Routing (CIDR), Variable Length Subnet Masking (VLSM) and route summarization, the concept of subnet masking by NOT using the default mask for that address class becomes more commonplace.
BTW, the answer to your test question is '1'. do you need an explanation why?
Cheers
Hippo
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| JayDot 2002-02-21, 11:09 am |
| I Understand The Answer
They Explained it~
'But didnt know why or when I would use Another Class Subnet Mask~
So Basically I could Use The Host Portion for More Networks/Subnets Whe I use Class B or Class C address on Class A IP...
It Does Make Sense..
Class C Subnet would Provide me More Subnets against a Class A and Class B Adrress...
Thanx~ | |
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| The Reamer 2002-02-21, 1:12 pm |
| You can use any class subnet for your needs. By borrowing bits from the host portion of the address, you extend the number of networks you have to work with.
Your company may have been assigned an IP address by your ISP. The ISP knows nothing of your setup. The address given to you could be large enough to handle all of the hosts you need on your network, but is this a wise choice. This is one of the things that we as engineers/administrators have to consider. Is my network running efficiently? Are there too many collisions that are taking place?
By using the different masks, you can break up the network into subnets that are more managable.
Now to the question you have listed. The subnetting is taking place in the 3rd octet. The 240 in the mask lets you know the networks run in increments of 16. If you list out the networks:
0
16
32
48
64
80
96
112....
You see that the ip address would fall between 96 and 112.
HTH,
Reamer | |
| The Reamer 2002-02-21, 3:13 pm |
| 
Just call me a bonehead. I started my reply when there were no followups to your question. I walked away for a while. Then finished and submitted the post without checking to see if there were any other responses. No need to be redundant.
Reamer | |
| JayDot 2002-02-21, 9:39 pm |
| Thanx~
For the Reply it all Makes A lot of Sense Now! |
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