ZyXEL NBG6503 User Manual download pdf (Page 175)

Appendix A IP Addresses and Subnetting

NBG6503 User’s Guide

175

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

networks, A and B.

Figure 123 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 75 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 ... 170 171 172 173 174 175 176 177 178 179 180 ... 239 240

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ZyXEL NBG6503 User Manual Page 175