Please Whitelist This Site?
I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)
If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.
If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.
Thanks for your understanding!
Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide
NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.
IP Subnetting Step #3: Determining The Custom Subnet Mask
(Page 2 of 2)
Class B Custom Subnet Mask Calculation Example
Now, let's do the same example with
our Class B network (184.108.40.206) with 5 bits for the subnet ID (with
a bit less narration this time; see Figure 77):
Figure 77: Determining The Custom Subnet Mask for A Class B Network
- Determine Default Subnet Mask:
For Class B, the subnet mask is 255.255.0.0. In binary, this is:
11111111 11111111 00000000
- Change Left-Most Zeroes To Ones
For Subnet Bits: We have decided to use 5 bits for the subnet ID,
so, we change the 5 left-most 0 bits from a 0 to 1, shown highlighted
below, to give us our binary custom subnet mask:
11111111 11111111 11111000
Determining The Custom Subnet Mask Using Subnetting Tables
- Convert Subnet Mask To Dotted Decimal
Notation: We take each of the octets in the subnet mask and convert
it to decimal, to give us a custom subnet mask of 255.255.248.0
- Express Subnet Mask In Slash
Notation: We can express the subnet mask 255.255.248.0 as
/21, since it is 21 ones followed by 11 zeroes. In other
words, its prefix length is 21.
Now, what could be easier than that?
Well, you could simply refer
to the subnetting summary tables. Find
the table for the appropriate class, and then find the row that you
selected in the previous step that matches the number of subnet ID bits
you want to use. You can see the matching subnet mask right there.
(Hey, it's good to know how to do
it yourself! You may not always have tables to refer to!)
|If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!|
Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.