Extending an IP Classful Address Using Subnet Masks

RFC 950, Internet Standard Subnetting Procedure, was written to address the IP address shortage. It proposed a procedure, called subnet masking, for dividing Class A, B, and C addresses into smaller pieces, thereby increasing the number of possible networks.

A subnet mask is a 32-bit value that identifies which address bits represent network bits and which represent host bits. In other words, the router does not determine the network portion of the address by looking at the value of the first octet; rather, it looks at the subnet mask that is associated with the address. In this way, subnet masks let you extend the usage of an IP address. This is one way of making an IP address a three-level hierarchy, as shown in Figure B-5.

Figure B-5 A Subnet Mask Determines How an IP Address Is Interpreted

Network

Host

Network

Subnet

Host

Based on Subnet Mask

To create a subnet mask for an address, use a binary 1 for each bit that you want to represent the network or subnet portion of the address, and use a binary 0 for each bit that you want to represent the node portion of the address. Note that the 1s in the mask are contiguous. The default subnet masks for Class A, B, and C addresses are as shown Table B-3.

Table B-3 IP Address Default Subnet Masks

Class

Default Mask in Binary

Default Mask in Decimal

A

11111111.00000000.00000000.00000000

255.0.0.0

B

11111111.11111111.00000000.00000000

255.255.0.0

C

11111111.11111111.11111111.00000000

255.255.255.0

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