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VigorSwitch P1280 User’s Guide 12
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.
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 -