Special IP addresses

Earlier in the chapter, we talked about the IP addresses 0.0.0.0 and 255.255.255.255. These are reserved addresses and are used to signify all IP and broadcast addresses, respectively.

♦ The 0.0.0.0 address is a way of saying "all networks" and is commonly seen when we define a default route in Linux.

♦ The 255.255.255.255 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 192.168.0.0 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 0.0.0.0

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 192.168.0.1, the broadcast address of 192.168.0.255 is used to broadcast to all machines in the 192.168.0.0 network. The term unicast refers to a one-to-one communication to a specific host. Therefore, if you communicated directly to 192.168.0.1, 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 192.168.0.0 network.

To sum up, you can say that the IP address of 192.168.0.1 has a network address of 192.168.0.0 and a broadcast address of 192.168.0.255.

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

available hosts.

B Using 130.1.0.0 as the network component, you have 65,025 (2A8*2A8) available hosts.

C Using 192.5.1.0 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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