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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: Practical Subnet Design and Address Determination Example 1 2 IP Subnetting Step #2: The Key Design Trade-off: Partitioning Network Address Host Bits

Assessing Future Needs During Requirements Analysis

We need to analyze the requirements above not only for the present network, but for the near future as well. The current values for these two numbers represent how the network needs to be designed today. However, designing only for the present is not a good idea.

Suppose we have exactly four subnetworks in our network now. In theory, we could use only two bits for the subnet ID, since 2² is 4. However, if our company is growing rapidly, this would be a poor choice. When we need to add a fifth subnet we'd have a problem!

Similarly, consider the growth in the number of hosts in a subnet. If the current largest subnet has 60 hosts, you don't want 6a bits for the host ID, because that limits you to 62 hosts. You can divide large subnets into smaller ones, but this may just mean unnecessarily additional work.

So, what is the “near future”? The term is necessarily vague, because it depends on how far into the future the organization wants to look. On the one hand, planning for several years' growth can make sense, if you have enough IP addresses to do it. On the other, you don't want to plan too far out, since changes in the short term may cause you to completely redesign your network anyway.

IP Subnetting: Practical Subnet Design and Address Determination Example 1 2 IP Subnetting Step #2: The Key Design Trade-off: Partitioning Network Address Host Bits