Class C Addresses
A Class C address begins with binary 110. Therefore, the lowest number
that can be represented is 11000000 (decimal 192), and the highest number
that can be represented is 11011111 (decimal 223). If an IPv4 address
contains a number in the range of 192 to 223 in the first octet, it is a
Class C address.
Class C addresses were originally intended
to support small networks; the first three octets of a Class C address
represent the network number, and the last octet may be used for hosts.
One octet for hosts yields 256 possibilities; after you subtract the all
0s network number and all 1s broadcast address; only 254 hosts may be
addressed on a Class C network. Whereas Class A and Class B networks prove
impossibly large (without subnetting), Class C networks can impose too
restrictive a limit on hosts.
With 2,097,152 total network addresses
containing a mere 254 hosts each, Class C addresses account for 12.5
percent of the Internet's address space. With Class A and B exhausted, the
remaining Class C addresses are all that is left to be assigned to new
organizations that need IP networks. The figure summarizes the ranges and
availability of three address classes used to address Internet hosts.
Class D Addresses
A Class D address begins with binary 1110 in the first octet. Therefore, the
first octet range for Class D addresses is 11100000 to 11101111, or 224 to
239. Class D addresses are not used to address individual hosts. Instead,
each Class D address can be used to represent a group of hosts called a
host group, or multicast group.
For example, a router configured to run
EIGRP joins a group that includes other nodes that are also running EIGRP.
Members of this group still have unique IP addresses from the Class A, B,
or C range, but they also listen for messages addressed to 224.0.0.10,
which is a Class D address. Therefore, a single routing update message can be
sent to 224.0.0.10, and all EIGRP routers will receive it. A single
message sent to several select recipients is called a multicast. Class D
addresses are also called multicast addresses.
A multicast is different from a broadcast.
Every device on a logical network receives a broadcast, whereas only
devices configured with a Class D address receive a multicast.
Class E Addresses
If the first octet of an IP address begins with 1111, then the address is
a Class E address. Class E addresses are reserved for experimental
purposes and should not be used for addressing hosts or multicast groups.
|