The Illustrated Network- P61:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices.:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been. | CHAPTER 22 Hypertext Transfer Protocol 569 and asking for Joe Smith. A URN is like asking for Joe Smith getting an answer from a resolver and going to the current address where good old Joe is found. Joe Smith is an example of a URN in the human namespace. Of course if this is to work properly there can only be one Joe Smith in the world. Any namespace that can be used to uniquely identify any type of resource can be used as a URN. But before you rush out to invent a URN system for automobiles for example keep in mind that designing URNs for new namespaces is not that easy. Each URN must be recognized by some official body or another and must be strictly defined by a formal language. It s not enough to say that the URN string will identify a car. It is necessary to define things such as the length of the string and just what is allowed in the string and what isn t actually there s a lot more to it than that . For example the International Standard Book Number ISBN system uniquely identifies books published all over the world. Part of the number identifies region of the world where the book is published another part the publisher yet another part the particular book and finally there is a checksum digit that is computed in case someone makes a mistake writing down one of the other parts. The formal definition of the ISBN namespace would establish the length of these fields and note that the ISBN must be 10 digits long and can only be made up of the digits 0 through 9 except for the last checksum digit where the Roman numeral X is used for the checksum 10 10 is a valid ISBN checksum digit . The general format of a URN is URN namespace-ID resource-identifier . Note the lack of any sense of location. The namespace ID is needed to distinguish a 10-digit telephone number from a 10-digit ISBN numbers for example and the URN literally makes it obvious that the URN notation system is being employed. Work on URNs has been slow. A resource identified by URN still has a .
