ZyXEL NBG4615 User Manual download pdf (Page 256)

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101

Appendix C IP Addresses and Subnetting

NBG4615 User’s Guide

256

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 168 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.

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

Table 96 Alternative Subnet Mask Notation (continued)

SUBNET MASK

ALTERNATIVE

NOTATION

LAST OCTET

(BINARY)

LAST OCTET

(DECIMAL)

1 2 ... 251 252 253 254 255 256 257 258 259 260 261 ... 363 364

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