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Class D and E:
Class D is a class with first 4 MSB (Most significance bit) set to 1-1-1-0 and
is used for IP Multicast. See also RFC 1112. Class E is a class with first 4 MSB set
to 1-1-1-1 and is used for IP broadcast.
According to IANA (Internet Assigned Numbers Authority), there are three
specific IP address blocks reserved and to be used for extending internal network.
We call them private IP addresses and they are listed below:
Class A 10.0.0.0 --- 10.255.255.255
Class B 172.16.0.0 --- 172.31.255.255
Class C 192.168.0.0 --- 192.168.255.255
Please refer to RFC 1597 and RFC 1466 for more information.
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 the IP network management task.
For a class B network, 128.1.2.3, it may have a subnet mask 255.255.0.0 by
default, in which the first two bytes are 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 are 1s. It’s now clear that the first two bytes are 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 ones which their host number are all zeros
and all ones.
or example, an IP address 128.1.2.128/25; to represent the network
itself, the IP address is 128.1.2.128; and for IP broadcasting, the address used
would be 128.1.2.255.
10000000.00000001.00000010.1 0000000
12
8.1.2.128/25
25 bits
1 0000000
1 1111111
All 0s = 128.1.2.128
All 1s= 128.1.2.255
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