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 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 Subnet Addressing ("Subnetting") Concepts

IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing 1 2 3 4 IP Default Subnet Masks For Address Classes A, B and C

Subnet Mask Notation

Like IP addresses, subnet masks are always used as a 32-bit binary number by computers. And like IP addresses, using them as 32-bit binary numbers is difficult for humans. Therefore, they are usually converted to dotted decimal notation for convenience, just like IP addresses are.

Let's take a quick example to show what this is all about. Suppose we have the Class B network 154.71.0.0. We decide to subnet this using 5 bits for the subnet ID and 11 bits for the host ID. In this case, the subnet mask will have 16 ones for the network portion (since this is Class B) followed by 5 ones for the subnet ID, and 11 zeroes for the host ID. That's “11111111 11111111 11111000 00000000” in binary, with the bits corresponding to the subnet ID highlighted. Converting to dotted decimal, the subnet mask would be 255.255.248.0. Figure 66 illustrates this process.

Figure 66: Determining the Subnet Mask of a Subnetted Network

The class B network from Figure 65 has been shown at top, with 5 bits assigned to the subnet ID and 11 bits left for the host ID. To create the subnet mask, we fill in a 32-bit number with “1” for each network ID and subnet ID bit, and “0” for each host ID bit. We can then convert this to dotted decimal.

IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing 1 2 3 4 IP Default Subnet Masks For Address Classes A, B and C

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.