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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 Subnetting: Practical Subnet Design and Address Determination Example

IP Subnetting Step #1: Requirements Analysis 1 2 3 IP Subnetting Step #3: Determining The Custom Subnet Mask

Class C Subnetting Design Example

Let's take an example. Suppose we have a Class C network, base address 211.77.20.0, with a total of 7 subnets. The maximum number of hosts per subnet is 25. Looking at the subnetting summary table for Class C, the answer is instantly clear: we need 3 bits for the subnet ID. Why? This allows us 8 subnets and 30 hosts per subnet. If we try to choose 2 bits, we can't define enough subnets (only 4). As Figure 74 shows, if we choose 4 bits for the subnet ID, then we can only have 14 hosts per subnet.

Figure 74: Example Class C Subnetting: An “Easy Decision”

In this particular example, where 7 subnets are needed and 25 hosts are needed for the largest subnet, there is only one choice of subnet ID size that meets the requirements.

IP Subnetting Step #1: Requirements Analysis 1 2 3 IP Subnetting Step #3: Determining The Custom Subnet Mask

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