Subnetting Question

Blang008

Ok, I have my subnetting down pretty good. I can do most of them in under 1 min. The ones that I am having problems with are the larger numbers. For example I can subnet 172.20.195.63 255.255.252.0, but it takes me a little bit longer to find out. I know my Increment is going to be 4. But it takes me some time to write out and figure what the Subnet is using increments of 4. Does anybody have a quicker way to arrive at the answer?

dynamik

I'm not exactly sure what you're asking. Are you trying to figure out a quick way to find the increment or are you having trouble with the ranges?

If you want to find the increment, it's simple with binary.

252 = 11111100

The 00 would be what you are looking at here. Two bits will be four. Three will be eight, etc.

If you need to find which range in IP falls in, you do a binary AND of the IP and the subnet mask. See my example here: http://techexams.net/forums/viewtopic.php?p=248510#248510

Blang008

I can arrive at the increment with no problem. I was having trouble getting the range that the IP was in quickly. But looking at your previous post is it safe to assume that you just match the 1's in the IP and subnet mask to get the Subnet for that IP.

dynamik

Yes. It's called a "binary AND" operation. It's used to find the network address when given an IP address and subnet mask. You only need to do it on the octet that isn't 255 or 0. The octets that have 255 for the mask will be the same as the IP and the octets that have 0 will be 0. Just google something like subnet binary and. There's tons of examples.

Blang008

Dynamik...thanks for the help. I think I have finally got subnetting down. I was wondering though..How fast do you need to do this for the CCENT/CCNA. I can do most of them fairly fast and most I can do in my head. A couple of the longer ones with Class A and Class B I need to write some things down, but can still do them fairly quickly, around 45 sec to a min. I'm using CBT nuggets way to subnet by the way.

Essendon

Blang, seems like people suggest being able to do subnetting in about a minute in your head. I guess not more than a couple of minutes in any case, you know the time pressure you are under on a Cisco test.

jibbajabba

Just found a very nice online tool to test yourself (and getting faster in the process)

http://faculty.valleycollege.net/rpowell/jscript/subnet2.htm

Essendon

Gomjaba wrote:

Just found a very nice online tool to test yourself (and getting faster in the process)

http://faculty.valleycollege.net/rpowell/jscript/subnet2.htm

Added to my favorites! Good find there, Gomjaba.