Waters Network Systems GSM2108 Operating Manual page 18

networks.
For a class B network, 128.1.2.3, the subnet mask 255.255.0.0 in default, in which the first two
bytes are all 1s. This means more than 60 thousands of nodes in flat IP address will be at the
same network. This is too large to manage practically. Now if we divide it into a smaller network
by extending network prefix from 16 bits to, say 24 bits, its third byte is used to subnet this class
B network. Now it has a subnet mask 255.255.255.0, in which each bit of the first three bytes is
1. It's now clear that the first two bytes is used to identify the class B network, the third byte is
used to identify the subnet within this class B network and, of course, the last byte is the host
number.
Not all IP addresses are available in the sub-netted network.
reserved. They are the addresses with all zero's and all one's. For example, an IP address
128.1.2.128, what will the reserved IP address look like? All 0s mean the network itself, and all
1s mean IP broadcast.
128.1.2.128/25
10000000.00000001.00000010.1 0000000
In this diagram, the subnet mask with 25-bit long, 255.255.255.128, contains 126 members in
the sub-netted network. The length of network prefix equals the number of the bit with 1s in that
subnet mask. With this, you can easily count the number of IP addresses matched. The
following table shows the result.
Waters Network Systems
Network Subne
25 bits
All 0s = 128.1.2.128
All 1s= 128.1.2.255
User's Manual
GSM-2108/GSM-1008SFP
Two special addresses are
1 0000000
1 1111111
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