ZyXEL Communications NBG-417N, User Manual, Page: 227 / 279

Appendix C IP Addresses and Subnetting
User
’
s Guide
228
The following figure shows the company network after subnetting. There are now
two sub-networks,
A and B .
Figure 126 Subnetting Example: After Subnetting
In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of
2
7
– 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.
11 000000) or 255.255.255.192.
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