System information

SIGNAMAX a.s.
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22
Subnet mask:
It means the sub-division of a class-based network or a CIDR block. The subnet is
used to determine how to split an IP address to the network prefix and the host address in
bitwise basis. It is designed to utilize IP address more efficiently and ease to manage IP
network.
For a class B network, 128.1.2.3, it may have a subnet mask 255.255.0.0 in default, in
which the first two bytes is with all 1s. This means more than 60 thousands of nodes in flat IP
address will be at the same network. It’s too large to manage practically. Now if we divide it
into smaller network by extending network prefix from 16 bits to, say 24 bits, that’s using its
third byte 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 address is available in the sub-netted network. Two special addresses are
reserved. They are the addresses with all zero’s and all one’s host number. For example, an
IP address 128.1.2.128, what IP address reserved will be looked like? All 0s mean the
network itself, and all 1s mean IP broadcast.
In this diagram, you can see the subnet mask with 25-bit long, 255.255.255.128,
contains 126 members in the sub-netted network. Another is that 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.
Prefix Length No. of IP matched No. of Addressable IP
/32 1 -
/31 2 -
/30 4 2
10000000.00000001.00000010.1 0000000
128.1.2.128/25
25
b
its
1 0000000
1 1111111
All 0s = 128.1.2.128
All 1s= 128.1.2.255
Subnet
N
etwork