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

When you are building or upgrading a network as a whole, the first step isn't buying hardware, or figuring out protocols, or even design. It's requirements analysis, the process of determining what it is the network needs to do. Without this foundation, you risk implementing a network that may perfectly match your design—but not meet the needs of your organization. The exact same rule applies to subnetting as well. Before we look at the gory details of host addresses and subnet masks, we must decide how to subnet the network. To do that, we must understand the requirements of the network.

Analyzing the requirements of the network for subnetting isn't difficult, because there are only a few issues that we need to consider. Since requirements analysis is usually done by asking questions, here's a list of the most important questions in analyzing subnetting requirements:

• What class is our IP address block?
  
  
• How many physical subnets are on the network today? (A “physical subnet” generally refers to a broadcast domain on a LAN; a set of hosts on a physical network bounded by routers.)
  
  
• Do we anticipate adding any more physical networks in the near future, and if so, how many?
  
  
• How many hosts do we have in the largest of our subnets today?
  
  
• How many hosts do we anticipate having in the largest subnet in the near future?

The first question is important because everything in subnetting is based around dividing up a Class A, Class B or Class C network, so we need to know which we are dealing with. If we are in the process of designing a network from scratch and don't have a Class A, B or C block yet, then we will determine which we need based on the approximate size of the organization. After that, we need to determine two key numbers: how many physical subnets we have, and the maximum number of hosts per subnet.

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