ZyXEL Communications P-660 series Guia do Utilizador descarregar pdf (Página 241)

P-660R-F1 Series User’s Guide

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Subnet masks are expressed in dotted decimal notation just like IP addresses. The “natural” masks for class A,

B and C IP addresses are as follows.

Subnetting

With subnetting, the class arrangement of an IP address is ignored. For example, a class C address no longer

has to have 24 bits of network number and 8 bits of host ID. With subnetting, some of the host ID bits are

converted into network number bits.

By convention, subnet masks always consist of a continuous sequence of ones beginning from the leftmost bit

of the mask, followed by a continuous sequence of zeros, for a total number of 32 bits.

Since the mask is always a continuous number of ones beginning from the left, followed by a continuous

number of zeros for the remainder of the 32 bit mask, you can simply specify the number of ones instead of

writing the value of each octet. This is usually specified by writing a “/” followed by the number of bits in the

mask after the address.

For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with mask 255.255.255.128.

The following table shows all possible subnet masks for a class “C” address using both notations.

The first mask shown is the class “C” natural mask. Normally if no mask is specified it is understood that the

natural mask is being used.

Table 102 “Natural” Masks

CLASS NATURAL MASK

A 255.0.0.0

B 255.255.0.0

C 255.255.255.0

Table 103 Alternative Subnet Mask Notation

SUBNET MASK SUBNET MASK “1” BITS LAST OCTET BIT VALUE DECIMAL

255.255.255.0 /24 0000 0000 0

255.255.255.128 /25 1000 0000 128

255.255.255.192 /26 1100 0000 192

255.255.255.224 /27 1110 0000 224

255.255.255.240 /28 1111 0000 240

255.255.255.248 /29 1111 1000 248

255.255.255.252 /30 1111 1100 252

1 2 ... 236 237 238 239 240 241 242 243 244 245 246 ... 267 268

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