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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 Subnetting: "Three-Level" Hierarchical IP Subnet Addressing 1 2 3 4 IP Default Subnet Masks For Address Classes A, B and C

Applying the Subnet Mask: An Example

Now, let's see how the subnet mask might be used. Suppose we have a host on this network with an IP of 154.71.150.42. A router needs to figure out which subnet this address is on. This is done by performing the masking operation shown in Table 50 and Figure 67.

This result, 154.71.144.0, is the IP address of the subnet to which 154.71.150.42 belongs. There is no need to explicitly differentiate the network ID bits from the subnet ID bits, because we are still using “classful” addresses. Any router can see that since the first two bits of the address are “10”, this is a Class B address. So the network ID is 16 bits, and this means the subnet ID must be bits 17 to 21, counting from the left. Here, the subnet is the portion highlighted above: “10010”, or subnet #18. (I'll explain this better in the section on custom subnetting.)

IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing 1 2 3 4 IP Default Subnet Masks For Address Classes A, B and C