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CCNA Exam: Router's configuration


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
R1(config-if)#no shutdown
R1(config)#int s0/0
R1(config-if)#ip address
R1(config-if)#no shutdown
  /  overlaps with FastEthernet0/0
4)The given answer: The subnet mask of the FastEthernet interface should be changed to
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
6) Thank you

2 Solutions
ip address specifies that the address is part of the subnet
24 mask bits =
28 mask bits = subnet has a host range of to (0 being network and 255 being broadcoast) subnet has a host range of to (16 being network and 31 being broadcast) subnet has a host range of - (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 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.

The two subnets overlap.

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

Int s0/0's ip subnet range is from - (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.

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

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