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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 Subnet Addressing ("Subnetting") Concepts

Previous Topic/Section
IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing
Previous Page
Pages in Current Topic/Section
1
2
34
Next Page
IP Default Subnet Masks For Address Classes A, B and C
Next Topic/Section

IP Subnet Masks, Notation and Subnet Calculations
(Page 2 of 4)

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.

 


Previous Topic/Section
IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing
Previous Page
Pages in Current Topic/Section
1
2
34
Next Page
IP Default Subnet Masks For Address Classes A, B and C
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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