Appendix B IP Addresses and Subnetting

Table 17 Alternative Subnet Mask Notation (continued)

SUBNET MASK

ALTERNATIVE

LAST OCTET

LAST OCTET

NOTATION

(BINARY)

(DECIMAL)

 

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.

The following figure shows the company network before subnetting.

Figure 52 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.

The following figure shows the company network after subnetting. There are now two sub- networks, A and B.

66

 

STB-1001S2 User’s Guide