Understanding the network.pdf - Back to Home

The limitations of the IP address space were only realized later in IPv4's evolution.
What this realization means to you as an IP networker is that you need to
understand two methods of implementing IP addressing. The original method is
known as classful addressing, and the revised method is known as classless
addressing.
Classful IP Addressing
The earlier statement that an IP address has two parts is only half true. All IP
addresses have two parts: the network and node address. There is, however, a
second address known as the subnet mask that is required to make sense of the IP
address.
TCP/IP is based on the idea that a collection of independent networks are connected
together by gateways, and that any node on a network can connect to any other
node by knowing only the node's IP address. To make this possible, routing is used
to send IP datagrams between different networks. To make it easy to distinguish
between the node address and the network address, IP's creators made different IP
address classes. There are three common classes of IP address space (A, B, and C)
and a defined number of networks available in each class. The class type determines
how many bits can be used to define the network address and how many bits can be
used for the node address. The way to determine the class of the network is to look
at the placement of the first significant bit. To do this, you must look at the address
in binary format.
If the first bit is 0, it is a Class A network, which can support up to 16 million hosts.
If the first bit is 1 and the second bit is 0, it is a Class B network, which can support
up to 64,000 hosts. If the first two bits are 1s and the third bit is 0, it is a Class C
network, which can support 254 hosts.
There are also Class D and Class E address spaces. Class D is reserved for use by
multicast services provided by IGMP, the Internet Group Management Protocol.
Multicast broadcasts allow a single IP datagram message to be sent to specific
groups of hosts that belong to the multicast group. Multicasts can be used across a
single subnet, or they can be forwarded across IP network segments or subnets by
setting up a router to perform multicast forwarding. Multiple multicast groups can
exist on one subnet. The destination host addresses are specified in the address
field of the multicast IP datagram. The Class E space is not used at all in networks.
It was reserved for testing and development. Most networking equipment will reject
addresses from the Class E space. See Figure 2.3 for a comparison of the networks.