Special IP addresses

Earlier in the chapter, we talked about the IP addresses and These are reserved addresses and are used to signify all IP and broadcast addresses, respectively.

♦ The address is a way of saying "all networks" and is commonly seen when we define a default route in Linux.

♦ The address is a catchall address that is called a broadcast address. All IP addresses on a network will listen to this address, as well as their own IP address for broadcast traffic.

♦ The address (in the example we are discussing) is called the network address and again is reserved for internal use in TCP/IP. This is the same as the

address, but refers to the specific network as opposed to all networks.

The term broadcast is used to describe a way of communicating with many machines simultaneously on a network. In the case of, the broadcast address of is used to broadcast to all machines in the network. The term unicast refers to a one-to-one communication to a specific host. Therefore, if you communicated directly to, you would be performing a unicast operation. The term multicast refers to a broadcast to a selected group of hosts, such as all hosts on the network.

To sum up, you can say that the IP address of has a network address of and a broadcast address of

In Table 6-2 we talked about the number of hosts per network. We take this a step further now and specify based on the network mask how many hosts are available in each network (see Table 6-4).

Table 6-4: Network Class and Host Allocation

Class Hosts Available

A Using as the network component, you have 16,581,375 (2A8*2A8*2A8)

available hosts.

B Using as the network component, you have 65,025 (2A8*2A8) available hosts.

C Using as the network component, you have 255 (2A8) available hosts.

Remember that .255 and .0 are reserved, so the actual number of hosts available is two less than those stated.

If an organization has been given a Class A network for its use, it has an awful lot of hosts it can use. It takes a lot to be allocated a Class A address and is normally reserved for Internet service providers (ISPs). Even then, it would have to be an extremely large organization to justify the allocation of over 16 million public IP addresses. Most organizations have Class B or Class C networks.

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