Zyxel-communications G.SHDSL.bis 4-port Security Gateway P-793H Manual de usuario descargar pdf (Pagina 395)

P-793H User’s Guide

Appendix F IP Addresses and Subnetting 395

Subnet Masks

A subnet mask is used to determine which bits are part of the network number, and which bits

are part of the host ID (using a logical AND operation).

A subnet mask has 32 bits. If a bit in the subnet mask is a “1” then the corresponding bit in the

IP address is part of the network number. If a bit in the subnet mask is “0” then the

corresponding bit in the IP address is part of the host ID.

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 left most 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.

Table 142 “Natural” Masks

CLASS NATURAL MASK

A 255.0.0.0

B 255.255.0.0

C 255.255.255.0

Table 143 Alternative Subnet Mask Notation

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

255.255.255.0 /24 0000 0000

255.255.255.128 /25 1000 0000

255.255.255.192 /26 1100 0000

255.255.255.224 /27 1110 0000

255.255.255.240 /28 1111 0000

1 2 ... 390 391 392 393 394 395 396 397 398 399 400 ... 431 432

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Zyxel-communications G.SHDSL.bis 4-port Security Gateway P-793H Manual de usuario Pagina 395