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CCNA Exam: Router's configuration

Posted on 2009-05-14
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Last Modified: 2012-08-14
Hi,

1) This is related to the CCNA Exam.
2) This is related to the discussion at one of the prep test
3) There is an error of this router's configuration:
R1#config t
R1(config)#int fa0/0
R1(configi-if)#ip address 192.168.1.17 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#int s0/0
R1(config-if)#ip address 192.168.1.65 255.255.255.240
R1(config-if)#no shutdown
  / 192.168.1.0  overlaps with FastEthernet0/0
4)The given answer: The subnet mask of the FastEthernet interface should be changed to 255.255.255.240
5)My question: Please say it with your own wordings, but you can give any "links" if necessary: i) Could you show me why we have to change the subnet mask of the FastEthernet interface to 255.255.255.240
6) Thank you

tjie
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Question by:tjie
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ludo_friend earned 260 total points
ID: 24391043
ip address 192.168.1.17 255.255.255.0 specifies that the address is part of the 192.168.1.0/24 subnet
(fyi)
24 mask bits = 255.255.255.0
28 mask bits = 255.255.255.240
192.168.1.0/24 subnet has a host range of 192.168.1.1 to 192.168.1.254 (0 being network and 255 being broadcoast)
192.168.1.17/28 subnet has a host range of 192.168.1.17 to 192.168.1.30 (16 being network and 31 being broadcast)
192.168.1.65/28 subnet has a host range of 192.168.1.65 - 192.168.1.78 (64 being network and 79 being broadcast)

if fa0/0 was left as /24, you can see that the host range includes the host range of 192.168.1.64/28 which conflicts. simply speaking, you can't overlap the subnets.  

have a play with the subnet calculator to get an idea of how subnetting affects your ranges.
http://www.subnet-calculator.com/




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by:that1guy15
that1guy15 earned 240 total points
ID: 24391064
The two subnets overlap.

Int f0/0's ip subnet range is from 192.168.1.0 - 192.168.1.255 (.1 is the subnet # and .255 is the broadcast).

Int s0/0's ip subnet range is from 192.168.1.64 - 192.168.1.79 (64 is the subnet # and 79 is the broadcast).

A router can not have two interfaces in the same subnet range.

Here is a link to a subnet calculator to help you determine ranges and subnetting.
http://www.subnet-calculator.com/

My recommendation though is you will need to be able to subnet quickly in your head for the CCNA.
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