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VigorSwitch P2260 User’s Guide
18
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.