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Home > Archive > 70-216 > January 2002 > Understanding CIDR
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Understanding CIDR
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| iceman2001 2002-01-25, 5:26 am |
| Hi all,
I had this question in Transcenders.
Q:Your network consists of 200 computers on a single network segment.You have decided to install a router and split your network into 4 subnets of 50 computers each.You have been using the network IP address of 192.168.7.0 with the default subnet mask,and you want
to continue using the same network IP address.
Which of the following is an appropriate network IP address for one of the new subnets?
A:192.168.7.0/2
B:192.168.7.64/24
c:192.168.7.128/25
d:192.168.7.192/26
Ans is d:
Transcenders explained in a short way but can
anyone direct me to a site that explain CIDR,or even explain your own logic.
I really need to nail an understanding of this!!!!
Cheers
iceman | |
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| unreal 2002-01-25, 9:44 am |
| hi, maybe this will guide you:
11111111 11111111 11111111 11xxxxxx
slash 26 means NIC is controlling the top 26 bits, indicated by the 1s.
So you have only 6 bits to play with, which for a class C network, gives you 2 to the power of 6 = 64 hosts per subnets.
Below is the work out.
Ip address mask No of hosts
1. 192.168.7.0 255.255.255.192 64
Range 192.168.7.1 to 192.168.7.62
Broadcast 192.168.7.63
2. 192.168.7.64 255.255.255.192 64
Range 192.168.7.65 to 192.168.7.126
Broadcast 192.168.7.127
3. 192.168.7.128 255.255.255.192 64
Range 192.168.7.129 to 192.168.7.190
Broadcast 192.168.7.191
4. 192.168.7.192 255.255.255.192 64
Range 192.168.7.193 to 192.168.7.254
Broadcast 192.168.7.255
So you can see answer D is correct as it is found in the 4th subnet counting from the top.
Good Luck ! | |
| iceman2001 2002-01-25, 10:08 am |
| Thanks for the help guys.The fog is lifting on this area already ;-))
iceman | |
| Wilbur 2002-01-25, 1:00 pm |
| Go to Freak's site and pull down his subnetting guide. It was a big help to me for all the tests.
www.mcsefreak.com | |
| Psydefx 2002-01-25, 1:24 pm |
| Classfull subnetting has several limitations, the most apparent of which is using the high order bits to distinguish addresses. The reason CIDR is so much more dynamic is because it allows the router to identify the exact number of network bits for a given address and then pick the appropriate interface.
Think of going to a hotel room.
When you arrive at the hotel, the desk clerk gives you a room key and tells you to go to suite 2214. From this information (in most hotels) you immediately discern that you will be staying on the 22nd floor. Grab your bags and go to the elevator.
CHOOSE 22 from the control panel (range=LL-P)
When you arrive at the 22nd floor, you step out of the elevator and directly in front of you is a sign with room numbers and arrows.
CHOOSE: Follow the left arrow for rooms 2201-2249. (ranges= 2201-2249 Left, 2250-2299 Right)
You are now walking down the hall looking for a door with the number 2214. When you reach the door, you have the appropriate key to enter the room.
This isn't an exact parallel, but what CIDR allows a router to do is deliniate individual addresses by a number of network bits. When you punched the elevator button, you were looking at /16. When you looked at the sign, you had already gotten to the "netowrk" and were looking for the appropriate "subnetwork" /18, (or two more digits have been deleted from our area of focus).
Please let me know if this has helped.
 Cheers
steve@sosdesk.com |
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This should explain it better than my interpretation