CCNA Exam: Router's configuration

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
tjieAsked:
Who is Participating?
 
ludo_friendCommented:
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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that1guy15Commented:
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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