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Prestige 2602R Series User’s Guide

Appendix C IP Subnetting 294

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 87 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.

Table 88 Two Subnets Example

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

1 2 ... 289 290 291 292 293 294 295 296 297 298 299 ... 348 349

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ZyXEL Communications P-2602R-63 Betriebsanweisung Seite 294