Figure 15 shows that the 25 bit extended prefix assigns 9 bits to the

subnet number field. Since 29 = 512, nine bits allow the definition of

512 subnets. Depending on the organization's requirements, the net

work administrator could have elected to assign additional bits to the

host number field (allowing more hosts on each subnet) and reduce the

number of bits in the subnet number field (decreasing the total number

of subnets that can be defined).

Although this example creates a rather large number of subnets, it illus

trates what happens to the dotted decimal representation of a subnet

address when the subnet number bits extend across an octet boundary.

Note that the same type of confusion can occur when the host number

bits extend across an octet boundary.

Defining Each of the Subnet Numbers

The 512 subnets will be numbered 0 through 511. The 9 bit binary rep

resentation of the decimal values 0 through 511 are: 0 (0000000002 ), 1

(0000000012 ), 2 (0000000102 ), 3 (0000000112 ), ..., 511 (1111111112 ).

To define Subnet #3, the network administrator places the binary rep

resentation of 3 (0000000112 ) into the 9 bits of the subnet number

field. The 512 subnet numbers for this example are listed in the follow

ing sample code. The underlined portion of each address identifies the

extended network prefix, while the bold digits identify the 9 bits repre

senting the subnet number field:

Base Net: 10001100.00011001 .00000000.00000000 = 140.25.0.0/16

Subnet #0: 10001100.00011001.00000000.0 0000000 = 140.25.0.0/25

Subnet #1: 10001100.00011001.00000000.1 0000000 = 140.25.0.128/25

Subnet #2: 10001100.00011001.00000001.0 0000000 = 140.25.1.0/25

Subnet #3: 10001100.00011001.00000001.1 0000000 = 140.25.1.128/25

Subnet #4: 10001100.00011001.00000010.0 0000000 = 140.25.2.0/25

Subnet #5: 10001100.00011001.00000010.1 0000000 = 140.25.2.128/25

Subnet #6: 10001100.00011001.00000011.0 0000000 = 140.25.3.0/25

Subnet #7: 10001100.00011001.00000011.1 0000000 = 140.25.3.128/25

Subnet #8: 10001100.00011001.00000100.0 0000000 = 140.25.4.0/25

Subnet #9: 10001100.00011001.00000100.1 0000000 = 140.25.4.128/25

.

.

Subnet #510: 10001100.00011001.11111111.0 0000000 = 140.25.255.0/25

Subnet #511: 10001100.00011001.11111111.1 0000000 = 140.25.255.128/25

Note that the sequential subnet numbers are not sequential when

expressed in dotted decimal notation. This can be confusing to people

who expect dotted decimal notation to make IP addressing easier. In

this example, the dotted decimal notation obscures the subnet number

ing scheme.

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