Help about Subnet Masks?

First, there is no easy way. But, there are understandable ways to do this process manually. I will describe one here.

First, keep in mind that an IP address is made up of 4 octets of 8 bits each separated by periods. The subnet mask is a numericvalue applied to the IP address that instructs routers where to break the IP address into "network address" and "host address". You probably know that the mask 255.255.255.0 is a class C subnet mask identifying the first 3 octets as the network address and the 4th octet as the host address. Here is the way that determination is made.

Use the mask 255.255.255.192 as an example. In binary representation,this is 11111111.11111111.11111111.11000000. The 1's are the network address and the 0's are the host addresses. In this case, the first 3 octets are all 1's, denoting a class C subnet. Then, there are 2 additional 1's in the 4th octect representing a value of 192. (128 + 64). You must count these additional 1's from the right as if they were the end of an octet. With that in mind, the values of the additional 1's would be 3 (1 + 2). So, the result is a segmenting of the original class C into 4 subnets identified by the decimal value of the additional 1's. The decimal 3, indicates that you are breaking the network into 4 segments (0, 1, 2, 3). Next, you have 6 places in the 4th octet to establish the host addresses. Again, counting these values from the right, the maximum number of hosts would be binary 111111 or 64 (1 + 2 + 4 + 8 + 16 + 32). This is actually 63 + 0. The first address in binary is always 0.

So, in the above example, you would have 4 segments of 64 hosts each. Keep in mind that the first address in any segment is the network address and the last address is the broadcast address. So, you only have 62 usable addresses in each segment.

Check-out the WebSite http://www.learntosubnet.com/

Search the internet for the file netter.zip, an IP subnetting program if you want an easy route..

If you have a Cisco router, you should have a cd with Cisco's ConfigMaker on it. Under "Tools" it has an IP Subnet Calculator. I done use ConfigMaker for mutch other than this calcularor.

I posted a formula, a friend & I put together,on my ftp site http://www.mypage.onemain.com/thomasmarsh/
I used to "STRUGGLE" with it but now with this chart you can quickly go for max hosts or max subnets or what your host id will be and what the increments will be.
I love this formula-it's what i used when i certified on tcp/ip
i had such a hard time with the old formulas-i'm glad to share it with anyone who is having trouble.
Enjoy !!!
P.S. System needs time to replicate.

I think this tells the story much better than I ever could.

http://www.3com.com/nsc/501302.html

the microsoft press book out dec 2000
isbn 1-57231-904-6 or 0-73560989-6
is an excellent reference on windows2000 networking.

there are pdf's at the bottom of the page

I personally feel that trying to do subnet math by hand is error prone. I would goto www.net3group.com and download their IP Subnet Calculator. It works very well and has the functionality you like.

I developed a chart in Word or Text format that I used to calculate subnets and Variable Length subnets.

Send me an e-mail at don-willadsen@us.army.mil and I'll send it to you, and if you like it you can rate my answer.

Good fortune,
Don

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