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Select the correct base network ID for
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This is an email reply to a student
It is, however, a great tutorial on Network+, IP subnetting, or any Cisco problem. The student is referring to the Will Harper Cheat-sheet (and my mistake in the explanation). I'm including it for 2 reasons. It explains the answer to the problem - and it demonstrates a student who thinks for himself, in the face of contradictory information. To refer to the Will Harper IP Subnetting Cheat-sheet, use this link.
(Student) I need your help!...My ... daughter mistakenly took my notepad which I had class notes in and summarily lost it. In those notes we're my various explanations for sub-netting as I understood them. They worked and I knew I would have to refer to them for further study. Alas now they're gone. I was doing my homework last night at work and couldn't figure out for the life of me how to get to my answer. I went to the Internet and started reading RFC's on subnetting and visited several sites on the subject which only confused me to no end [emphasis added]. Therefore I need your explanation again as to how to use the chart to achieve my answers. IE:
Select the correct base network ID for 220.127.116.11 /22
I can't remember the formula (steps used to achieve the answer using your chart). Please help.
(Trainer) Select the correct base network ID for 18.104.22.168 /22 from the Will Harper IP Subnetting Cheat Sheet
The CIDR mask /22 is 2-bits into the B-hosts Octet. E.g. 255.255.248.0 Mask bits are 1111 1100 0000 0000 That means that the Subnet bits are The first 6 bits of the IP address and the host bits are the last 2 of the IP address IN THE 3rd Octet. (Can you find the error in this explanation? The student did.)
45, in bits is 64=0
Hence the bits in the IP address 45 are 0010 1101
Inspecting the mask, and noting that the "Subnet bits" are all BUT the last 2 bits
Hence the network IP is 0010 1100 0000 0000 or 44.0 (set all of the host bits to 0). Since /22 is masking in the 3RD OCTET, the first two octets always remain the same.
(Student) Looking at the [Will Harper IP Subnetting Cheat Sheet] chart going across, the mask correlates to 252...
(Trainer) Gotcha! I was just testing you (do you believe that?). The 252 mask in the chart is correct, NOT 248.
Sounds like you have pretty well mastered this lesson. The question, "Select the correct base network ID" is a simple matter of first, determining which bits in the MASK are host bits (by figuring out which ones are network bits); then reduce the "people" IP ADDRESS to bits; then identify the host bits in the IP address; then apply the simple rule, "the subnet IP address is the IP address you get when you set all of the HOST BITS in the IP ADDRESS to zeros." The final step is to convert the zero-host-IP bits back to a "people" IP Address, so you can type it in.
Will Harper, or
Roger W. Geyer
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Webmaster Will Harper, MCSE, MCT, CCNA 08/27/2009 22:39
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