option in Figure 16). In both cases, the requester
forms a query that includes the name to be
resolved, in addition to other fields. When a
server receives a request, it checks to see if the
name is within its area (in the data base). If so, it
translates the name into the address and appends
this result to the request before returning the
reply message. If the server is not able to resolve
the name, it forwards the request to the next
name server (if a complete resolution was indi-
cated in the request) and returns the result to the
requester, or replies its lack of information to the
requester (if no complete resolution was indi-
cated). To initiate this procedure, all machines
must know the address of at least one domain
name server.
In order to have more efficient name resolutions
(as well as to increase the availability of the res-
olution function), local name servers are com-
monly used. Such a local server may keep a list
of all names recently being resolved (recently
refers to a time less than a time-to-live which
could be set per name). Asking the local server
first would frequently make the resolution func-
tion more efficient as it turns out that more
requests are initiated for the same domain name.
Resolving a domain name is also commonly
referred to as name binding.As seen from the example, the names/addresses
are hierarchical. The last part of the name (after
the last period) tells which “area” out of the first/
top-level separation. For the IP-address the firstFigure 16 Examples of
sequences for name resolution.
Note that messages and
addresses are fictive1 Resolve(redcar.bt.com) 3 Resolved(159..................)2 Resolve(redcar.bt.com)4 Resolved(159..................)
5 IP packets redcar.bt.com
159...............redcar.bt.com
→159...............DNS server telenor.com domain DNS server bt.com domain
bt.com
→bt.com DNS server1 Resolve(redcar.bt.com)Redir(DNS server. 3 Resolve(redcar.bt.com)
bt.com address)2
4 Resolved(159..................)5 IP packets redcar.bt.com
159...............redcar.bt.com
→159...............DNS server telenor.com domain DNS server bt.com domain
bt.com
→bt.com DNS server1 Resolve(viking.telenor.com)3 IP packets2 Resolved(129...............)viking.telenor.com
→129...............DNS server telenor.com domainviking.telenor.com
→129...............