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Appendix B IP Addresses and Subnetting

VMG1312-B Series User’s Guide

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The following table shows some possible subnet masks using both notations.

Subnetting

You can use subnetting to divide one network into multiple sub-networks. In the following example

a network administrator creates two sub-networks to isolate a group of servers from the rest of the

company network for security reasons.

In this example, the company network address is 192.168.1.0. The first three octets of the address

(192.168.1) are the network number, and the remaining octet is the host ID, allowing a maximum

of 2

– 2 or 254 possible hosts.

The following figure shows the company network before subnetting.

Figure 182 Subnetting Example: Before Subnetting

You can “borrow” one of the host ID bits to divide the network 192.168.1.0 into two separate sub-

networks. The subnet mask is now 25 bits (255.255.255.128 or /25).

The “borrowed” host ID bit can have a value of either 0 or 1, allowing two subnets; 192.168.1.0 /25

and 192.168.1.128 /25.

Table 114 Alternative Subnet Mask Notation

SUBNET MASK

ALTERNATIVE

NOTATION

LAST OCTET

(BINARY)

LAST OCTET

(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 ... 317 318 319 320 321 322 323 324 325 326 327 ... 375 376

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ZyXEL Communications VMG5313-B30A Guide de l'utilisateur Page 322