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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 able to be used for extending internal network. We
call it Private IP address and list 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 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.
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
Subne
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