Zyxel-communications Internet Security Gateway ZyWALL 2 Series Manual do Utilizador descarregar pdf (Página 563)

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ZyWALL 2 Series User’s Guide

IP Subnetting I-3

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.

Chart I-4 Alternative Subnet Mask Notation

SUBNET MASK IP ADDRESS 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

255.255.255.248 /29 1111 1000

255.255.255.252 /30 1111 1100

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.

Example: Two Subnets

As an example, you have a class “C” address 192.168.1.0 with subnet mask of 255.255.255.0.

NETWORK NUMBER HOST ID

IP Address 192.168.1. 0

IP Address (Binary) 11000000.10101000.00000001. 00000000

Subnet Mask 255.255.255. 0

Subnet Mask (Binary) 11111111.11111111.11111111. 00000000

The first three octets of the address make up the network number (class “C”). You want to have two separate

networks.

1 2 ... 558 559 560 561 562 563 564 565 566 567 568 ... 613 614

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