Application Layer At long last we can ask the question - how does the user interface with the network?

Domain Name Service IP addresses are hard to remember. One of the primary application-layer protocols is the one that allows a user to type in a text string as a network address, and then translates that string to an IP address. DNS is this protocol. The problem, of course, is scale.

The Name Space In order for a mapping system to work, the correspondence between names and IP addresses must be 1:1 and unique. Each IP address should have only one name, and each name should point to only one IP address. Therefore, just like we divide IP addresses into address spaces, we divide domain names up into name spaces. The Internet does this hierarchically.

Domain Name Space The Internet has a tree-shaped hierarchy of domain names. Each node in the tree has a string associated with it (63 char max). The root node contains the null string. Every child of a given node must contain a unique string. The full domain name of a given IP address is the collection of strings from bottom to top. Combined with the previous restriction, this guarantees all domain names are unique.

The hierarchy The first set of children of the root are the top- level domains, e.g. com, gov, edu, mil, etc. This level includes all the country domains. As you travel down the tree, names get more specific. A fully qualified domain name is each of the strings along branch of the tree, separated by dots. DNS requires FQDNs to be submitted for mapping to IP addresses.

Name servers Once upon a time, the Internet was small enough that each host kept a host file that mapped names to IP addresses. Obviously, those days are long gone. It also is not feasible to maintain complete name information on just one or a few central name servers. The solution DNS adopts is to allow for a hierarchy of name servers that mirrors the hierarchy of name spaces.

A given name server has responsibility for a zone of the name space. A zone can be as small as a subnet of a LAN or as large as an entire top-level domain. Only the servers at the bottom of the hierarchy maintain complete name-IP mappings of the hosts in their zone. All other servers maintain references to the lower level servers they are connected to. There are several root servers scattered around the world. These keep references to the other servers they delegate authority to (usually top-level domain servers).

Name-address Resolution DNS works as a client/server application. A host needing resolution sends a request to the nearest DNS server. That server checks its database. If it has the required mapping it sends it back to the client. If not, the server either refers the client to another DNS server, or it kicks the request higher up the tree. If the request gets to a root server without being resolved, it gets sent back down the appropriate branch of the hierarchy until an answer is found. This is called recursive resolution In iterative resolution, the server gives the client the address of another server that might have the answer. The client repeats the request and either gets an answer or is directed to yet another server. Once an answer is found, it is stored in the server’s cache.