1.4 Internet Addresses

Every interface on an internet must have a unique Internet address (also called an IP address ). These addresses are 32-bit numbers . Instead of using a flat address space such as 1, 2, 3, and so on, there is a structure to Internet addresses. Figure 1.5 shows the five different classes of Internet addresses.

Figure 1.5. The five different classes of Internet addresses.

These 32-bit addresses are normally written as four decimal numbers, one for each byte of the address. This is called dotted -decimal notation. For example, the class B address of the author's primary system is 140.252.13.33.

The easiest way to differentiate between the different classes of addresses is to look at the first number of a dotted-decimal address. Figure 1.6 shows the different classes, with the first number in boldface.

Figure 1.6. Ranges for different classes of IP addresses.

It is worth reiterating that a multihomed host will have multiple IP addresses: one per interface.

Since every interface on an internet must have a unique IP address, there must be one central authority for allocating these addresses for networks connected to the worldwide Internet. That authority is the Internet Network Information Center, called the InterNIC. The InterNIC assigns only network IDs. The assignment of host IDs is up to the system administrator.

Registration services for the Internet (IP addresses and DNS domain names ) used to be handled by the NIC, at nic.ddn.mil. On April 1,1993, the InterNIC was created. Now the NIC handles these requests only for the Defense Data Network (DDN). All other Internet users now use the InterNIC registration services, at rs.internic.net.

There are actually three parts to the InterNIC: registration services (rs.internic.net), directory and database services (ds.internic.net), and information services (is.internic.net). See Exercise 1.8 for additional information on the InterNIC.

There are three types of IP addresses: unicast (destined for a single host), broadcast (destined for all hosts on a given network), and multicast (destined for a set of hosts that belong to a multicast group ). Chapters 12 and 13 look at broadcasting and multicasting in more detail.

In Section 3.4 we'll extend our description of IP addresses to include subnetting, after describing IP routing. Figure 3.9 shows the special case IP addresses: host IDs and network IDs of all zero bits or all one bits.