Defining the Subnet Mask / Extended Prefix Length

The first step in defining the subnet mask is to determine the number of

bits required to define the six subnets. Since a network address can

only be subnetted along binary boundaries, subnets must be created in

blocks of powers of two [2 (21), 4 (22), 8 (23), 16 (24), and so on]. Thus,

it is impossible to define an IP address block such that it contains

exactly six subnets. For this example, the network administrator must

define a block of 8 (23) and have two unused subnets that can be

reserved for future growth.

Since 8 = 23, three bits are required to enumerate the eight subnets in

the block. In this example, the organization is subnetting a /24 so it will

need three more bits, or a /27, as the extended network prefix. A 27 bit

extended network prefix can be expressed in dotted decimal notation

as 255.255.255.224. This notation is illustrated in Figure 12.

F I G U R E 1 2 . E x a m p l e # 1 D e f i n i n g t h e S u b n e t

M a s k / E x t e n d e d P re f i x L e n g t h

A 27 bit extended network prefix leaves 5 bits to define host addresses

on each subnet. This means that each subnetwork with a 27 bit prefix

represents a contiguous block of 25 (32) individual IP addresses. How

ever, since the all 0s and all 1s host addresses cannot be allocated, there

are 30 (25 2) assignable host addresses on each subnet.

Defining the Subnet Numbers

The eight subnets will be numbered 0 through 7. Throughout the

remainder of this paper, the XXX notation indicates the binary repre

sentation of the number. The 3 bit binary representation of the decimal

values 0 through 7 are: 0 (000 ), 1 (001 ), 2 (010 ), 3 (011 ), 4 (100 ), 5

(101 ), 6 (110 ), and 7 (111 ).

In general, to define Subnet #N, the network administrator places the

binary representation of N into the bits of the subnet number field. For

example, to define Subnet #6, the network administrator simply places

the binary representation of 6 (110 ) into the 3 bits of the subnet num

ber field.

U N D E R S TA N D I N G I P A D D R E S S I N G

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