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Appendix B IP Addresses and Subnetting

VMG1312-B Series User’s Guide

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The following figure shows the company network after subnetting. There are now two sub-

networks, A and B.

Figure 183 Subnetting Example: After Subnetting

In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of 2

– 2 or 126

possible hosts (a host ID of all zeroes is the subnet’s address itself, all ones is the subnet’s

broadcast address).

192.168.1.0 with mask 255.255.255.128 is subnet A itself, and 192.168.1.127 with mask

255.255.255.128 is its broadcast address. Therefore, the lowest IP address that can be assigned to

an actual host for subnet A is 192.168.1.1 and the highest is 192.168.1.126.

Similarly, the host ID range for subnet B is 192.168.1.129 to 192.168.1.254.

Example: Four Subnets

The previous example illustrated using a 25-bit subnet mask to divide a 24-bit address into two

subnets. Similarly, to divide a 24-bit address into four subnets, you need to “borrow” two host ID

bits to give four possible combinations (00, 01, 10 and 11). The subnet mask is 26 bits

(11111111.11111111.11111111.11000000) or 255.255.255.192.

Each subnet contains 6 host ID bits, giving 2

- 2 or 62 hosts for each subnet (a host ID of all

zeroes is the subnet itself, all ones is the subnet’s broadcast address).

Table 115 Subnet 1

IP/SUBNET MASK NETWORK NUMBER

LAST OCTET BIT

VALUE

IP Address (Decimal) 192.168.1. 0

IP Address (Binary) 11000000.10101000.00000001. 00000000

Subnet Mask (Binary) 11111111.11111111.11111111. 11000000

1 2 ... 318 319 320 321 322 323 324 325 326 327 328 ... 375 376

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ZyXEL Communications VMG5313-B30A Guide de l'utilisateur Page 323