Appendix D IP Addresses and Subnetting

Notation

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 subnet mask 255.255.255.128.

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

Table 100 Alternative Subnet Mask Notation

SUBNET MASK

ALTERNATIVE

LAST OCTET

LAST OCTET

NOTATION

(BINARY)

(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

 

 

 

 

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 28 – 2 or 254 possible hosts.

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P-870HN-51D User’s Guide