DNS - The Internet's Directory Service, Computer Networking

DNS - The Internet's Directory Service

We human beings can be identified in many ways. For example, we can be identified by the names that appear on our birth certificates. We can be identified by our social security numbers. We can be identified by our driver's license numbers. Although each of these identifiers can be used to identify people, within a given context, one identifier may be more appropriate than an other. For example, the computers at the IRS (the infamous tax collecting agency in the US) prefer to use fixed-length social security numbers rather than birth-certificate names. On the other hand, ordinary people prefer the more mnemonic birth-certificate names rather than social security numbers. (Indeed, can you imagine saying, "Hi. My name is 132-67-9875. Please meet my husband, 178-87-1146.")

Just as humans can be identified in many ways, so too can Internet hosts. One identifier for a host is its hostname. Hostnames -- such as cnn.com, www.yahoo.com, gaia.cs.umass.edu and surf.eurecom.fr -- are mnemonic and are therefore appreciated by humans. However, hostnames provide little, if any, information about the location within the Internet of the host. (A hostname such as surf.eurecom.fr, which ends with the country code .fr, tells us that the host is in France, but doesn't say much more.) Furthermore, because hostnames can consist of variable-length alpha-numeric characters, they would be difficult to process by routers. For these reasons, hosts are also identified by so-called IP addresses. We will discuss IP addresses in some detail in Chapter 4, but it is useful to say a few brief words about them now. An IP address consists of four bytes and

has a rigid hierarchical structure. An IP address looks like 121.7.106.83, where each period separates one of the bytes expressed in decimal notation from 0 to 127. An IP address is hierarchical because as we scan the address from left to right, we obtain more and more specific information about where (i.e., within which network, in the network of networks) the host is located in the Internet. (Just as when we scan a postal address from bottom to top we obtain more and more specific

information about where the residence is located). An IP address is included in the header of each IP datagram, and Internet routers use this IP address to route s datagram towards its destination.