IP Addresses and Domain Names

Internetworking routes IP datagrams according to the IP address, but humans find names easier to remember. This section briefly reviews the principles of IP addresses and provides an overview of how names are resolved to addresses.

What Is an IP Address?

Perhaps the easiest way to understand IP addresses is to look at the Internet as a global network. All networks that comprise the global network are just subnets. InterNIC provides the first level of subnetworking by dividing the global address space into classes that are assigned to organizations. The organizations are then responsible for subdividing their assigned address space to meet their network needs.

Figure 1.18 : IP addresses and subnet masks.
Figure 1.18 : IP addresses and subnet masks.

The IP address is a 32-bit number. To simplify the notation of addresses, divide this number into four octets and write the octets in a dotted-decimal format. Three types of IP addresses exist: network address, host address, and broadcast address. Because every host is part of a network, you divide the IP address into a network portion and a local host portion. When the local host portion is all zeros, it is a network address; all ones is a broadcast address. Anything else is a host address. However, the IP address itself contains no information about what constitutes the network portion versus the local host portion. The subnet mask provides this information. By convention, binary ones define the network portion, and zeros define the local host portion. Again, by convention, the ones must be contiguous to the left, and the remainder is zeros.

As mentioned previously, InterNIC splits the global address space into classes and then assigns the network address according to these divisions. Table 1.1 shows the breakdown of the address space.


Table Class Network Address Subnet Mask No. of Networks
table A 1-126 126
table B 128-191 16,384
table C 192-223 2,097,152
table D 224-254 (experimental)


As mentioned before, the designations shown in Table 1.1 represent assigned network addresses. The network manager for an organization is then responsible for additional subnetting, according to the requirements of their individual networks.

Special IP Addresses

Several special IP addresses also exist. For an Internet programmer, the most important special addresses are the local loopback address and the broadcast address. For the network administrator, the most important special addresses are those set aside for networks not connected to the Internet.

The local loopback address ( enables a client application to address a server on the same machine without knowing the address of the host. This address is often called the local host address. In terms of the TCP/IP protocol stack, the flow of information goes to the Network layer, where the IP protocol routes it back up through the stack. This procedure hides the distinction between local and remote connections.

Broadcast addresses enable an application to send a datagram to more than one host. The special address sends a “limited broadcast” to all hosts on this network. A “direct broadcast” uses the address form A.255.255.255B.B.255.255, or C.C.C.255 to send messages to all hosts on a particular class A, B, or C network. Finally, a broadcast to a particular subnet is to the address with all local host bits set to one.

RFC 1918 specifies an Internet “best current practice” for address allocation on private internets (intranets). For a network not connected to the Internet, or a network where all Internet traffic passes through a proxy server, the Internet Assigned Numbers Authority (IANA) reserved three blocks of IP address space: to, to, and to This block is equivalent to one class A address, 16 class B addresses, and 256 class C addresses.

Resolving Names to Addresses

In the early days of ARPAnet, a system resolved names to addresses using the hosts file. The Stanford Research International (SRI) maintained the hosts file, and each site periodically downloaded an updated copy of the file. As the number of sites connected to ARPAnet increased, this method proved too hard to maintain and placed an increasing burden on the network. In 1984 Paul Mockapetris, of University of Southern California’s Information Sciences Institute, released RFCs (882 and 883) that describe the domain name system. Today, DNS is the standard for resolving names to addresses. However, the hosts file still plays a role in name resolution during the booting of a system and as a means to provide LAN resolution when DNS is down.

Figure 1.19 : The hierarchical structure of DNS.

In a nutshell, DNS is a distributed database whose structure looks like the UNIX file system. DNS is a client/server system in which the resolvers query name servers to find an address record for a domain name. The query process begins with the root name servers. If the root name server does not know the answer, it returns the address of a name server that knows more details about the domain name. The resolver then queries the new name server. This iterative process continues until a name server responds with the address for the domain name.

The resolver maintains the retrieved information in a cache until the designated time to live (TTL) for the record expires. This approach reduces the number of queries and, at the same time, responds to the dynamic nature of networks. By distributing the database across the Internet, the site responsible for the information maintains the information.