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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 Default Subnet Masks For Address Classes A, B and C 1 2 3 4 IP Subnet Identifiers, Subnet Addresses and Host Addresses

Determining the Custom Subnet Mask

Once we have determined how many bits we are going to devote to the subnet ID and the host ID, we can figure out the subnet mask. This is fairly easy to do, now that we understand how subnetting works. We begin with the default subnet mask in binary for the appropriate class of our network. We then start with the left-most zero in that mask and change as many bits to 1 as we have dedicated to the subnet ID. We can then express the subnet mask in dotted decimal form. Figure 69 shows how the custom subnet mask can be determined for each of the subnetting options of a Class C network, in both binary and decimal.

So, to take the example in that figure, consider the Class C network 200.13.94.0. There are 8 bits in the original host ID, which gives us six different subnetting options (we can’t use 7 or 8 bits for the subnet ID, for reasons we will discuss shortly.) Suppose we use three of these for the subnet ID and five are left for the host ID. To determine the custom subnet mask, we start with the Class C default subnet mask:

11111111 11111111 11111111 00000000

We then change the first three zeroes to ones, to get the custom subnet mask:

11111111 11111111 11111111 11100000

In dotted decimal format, this is 255.255.255.224.

IP Default Subnet Masks For Address Classes A, B and C 1 2 3 4 IP Subnet Identifiers, Subnet Addresses and Host Addresses