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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 Subnet Identifiers, Subnet Addresses and Host Addresses 1 2 3 4 IP Variable Length Subnet Masking (VLSM)

Since there are only a few options for how to subnet each of Class A, Class B and Class C networks, I have listed the options for each class in three summary tables below: Table 52, Table 53 and Table 54. These tables can help you quickly decide how many bits to use for subnet ID and host ID, and then what the subnet mask is for their selection. They also summarize nicely what we've discussed in this section.

Each row of each table shows one possible subnetting option for that class, including the number of bits for each of the subnet ID and host ID, and the number of subnets and hosts based on the number of bits. I then show the subnet mask in binary and decimal form, as well as in CIDR notation. Finally, I include the formula for calculating the addresses for each subnet under each of the options.

A few additional explanatory notes are in order regarding these tables:

• The values for the number of subnets per network assume that the all-zeroes and all-ones subnets are allowed. If not, you must subtract 2 from those figures; this also means that the option using only one bit for the subnet ID becomes invalid, and the subnet address formulas no longer work as shown.
  
  
• The number of hosts per subnet does exclude the all-zeroes and all-ones cases, so it is two to the power of the number of host ID bits, less two.
  
  
• The first row of each table shows the “default” case where the number of subnet bits is zero, and thus the subnet mask is the default subnet mask for the class.
  
  
• In the subnet mask for all options but the default, I have highlighted the portion of the subnet mask corresponding to the subnet ID, for clarity. This has been done for each individual bit of the binary mask, and for each octet in the dotted decimal representation of the mask where part of the subnet ID is found.
  
  
• You will see that not all of the divisions make a great deal of sense in the real world… though you might be surprised. For example, at first glance it seems silly to think that we might want to assign 14 bits of a Class B host ID to the subnet ID and leave 2 bits for the host ID—what sort of real network has 16,384 subnets with 2 hosts on each? Yet some larger Internet service companies may indeed require thousands of tiny subnets when setting up connections between routers, or between their core network and their customers.
  
  
• The subnet address formulas in the last column of each table show the address for subnet #N (numbering from zero up to one less than the maximum number of subnets).See the end of step #4 in the step-by-step subnetting section for a full explanation of how these formulas work.

IP Subnet Identifiers, Subnet Addresses and Host Addresses 1 2 3 4 IP Variable Length Subnet Masking (VLSM)