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A subnet is only known and understood locally; to the rest of the Internet, the address is still interpreted as a standard IP address. Table 3.2 shows how this works for the standard IP address classes.

TABLE 3.2 Default Subnet Masks for Standard IP Address Classes


Subnet Mask Bit Pattern

Subnet Mask


11111111 00000000 00000000 00000000


11111111 11111111 00000000 00000000


11111111 11111111 11111111 00000000

Routers then use the subnet mask to extract the network portion of the address so that they can send the data packets along the proper route on the network.

Because all the Class A and Class B networks are taken, you are most likely to encounter subnet-related issues when working with a Class C network. In the next section, you’ll get a detailed look at how to subnet a Class C network.

Why Subnet?

When faced with the choice of whether or not to subnet your network, you must remember several of the advantages to subnetting. The following list summarizes the advantages of the subnetting solution.

■    It reduces the size of routing tables.

■    It minimizes network traffic.

■    It isolates networks from others.

   It maximizes performance.

   It optimizes IP address space.

   It enhances the ability to secure a network.

Subnetting a Class C Network

How do you find out the values that you can use for a Class C network subnet mask? Remember from a previous discussion that InterNIC defines the leftmost three octets in the address, leaving you with the rightmost octet for your own network addresses. If your network consists of a single segment, you have the following subnet mask:

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