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.

The Book is Here... and Now On Sale!

Enjoy The TCP/IP Guide? Get the complete PDF!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing
                          9  IP Subnetting: Practical Subnet Design and Address Determination Example

Previous Topic/Section
IP Subnetting Step #2: The Key Design Trade-off: Partitioning Network Address Host Bits
Previous Page
Pages in Current Topic/Section
1
2
Next Page
IP Subnetting Step #4: Determining Subnet Identifiers and Subnet Addresses
Next Topic/Section

IP Subnetting Step #3: Determining The Custom Subnet Mask
(Page 1 of 2)

Once we have decided how many bits to use for the subnet ID and how many to leave for the host ID, we can determine the custom subnet mask for our network. Now, don't go running for cover on me. J A lot of people's eyes glaze over at mention of the subnet mask, but it's really quite simple to figure out once we have done our homework in making the design decision we did in Step #2. In fact, there are two ways of doing this; one is less work than the other, but they're both quite easy. I was going to call them the “hard” way and the “easy” way, but instead, I'll call them “easy” and “easier”.

Calculating The Custom Subnet Mask

Let's start with the “easy” method, in which we determine the subnet mask in binary form from the information we already have about our network, and then convert the mask to decimal. To refresh your memory and guide the process, remember this: the subnet mask is a 32-bit binary number where a 1 represents each bit that is part of the network ID or subnet ID, and a 0 represents each bit of the host ID.

Class C Custom Subnet Mask Calculation Example

Refer back to the Class C example in the previous topic. We decided to use 3 bits for the subnet ID, leaving 5 bits for the host ID. Here are the steps we will follow to determine the custom subnet mask for this network (illustrated in Figure 76):


Figure 76: Determining The Custom Subnet Mask for A Class C Network

 


  1. Determine Default Subnet Mask: Each of Classes A, B and C has a default subnet mask, which is the subnet mask for the network prior to subnetting. It has a 1 for each network ID bit and a 0 for each host ID bit. For Class C, the subnet mask is 255.255.255.0. In binary, this is:
11111111 11111111 11111111 00000000
  1. Change Left-Most Zeroes To Ones For Subnet Bits: We have decided to use 3 bits for the subnet ID. The subnet mask has to have a 1 for each of the network ID or subnet ID bits. The network ID bits are already 1 from the default subnet mask, so, we change the 3 left-most 0 bits in the default subnet mask from a 0 to 1, shown highlighted below. This results in the following custom subnet mask for our network:
11111111 11111111 11111111 11100000
  1. Convert Subnet Mask To Dotted Decimal Notation: We take each of the octets in the subnet mask and convert it to decimal. The result is our custom subnet mask in the form we usually see it: 255.255.255.224.

  2. Express Subnet Mask In “Slash Notation”: Alternately, we can express the subnet mask in “slash notation”. This is just a slash followed by the number of ones in the subnet mask. 255.255.255.224 is equivalent to “/27”.

Previous Topic/Section
IP Subnetting Step #2: The Key Design Trade-off: Partitioning Network Address Host Bits
Previous Page
Pages in Current Topic/Section
1
2
Next Page
IP Subnetting Step #4: Determining Subnet Identifiers and Subnet Addresses
Next Topic/Section

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!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - 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.