Name resolution refers to the process of converting host names or domain names to an IP address. On an IP network, computers communicate using the IP address; however, computers are assigned names which are easy to remember. When a user attempts to access a computer by using the computer or hostname, it is automatically translated to the IP address assigned to it and then, the communication takes place.
Several methods are used for name resolution as described below:
- Using a HOSTS file
- Using DNS
- Using WINS
- Using DDNS
The hosts file is a computer file used by the operating system to map host names to IP addresses. The hosts file is a plain text file and is conventionally named hosts.
Note: Though it's a plain text file, this file does not have extension as .TXT.
HOSTS file contains lines of text mapping hostnames to IP addresses. HOSTS use a simple mechanism of separating hostnames and IP addresses by white space or tabs; this is very similar to that of a phonebook having entries of customers and their phone numbers. You can map friendly names such as John, PRINTSERVER, etc. that will be easier for users to remember; however for the computers to identify and connect, you need to map the name with respective IP address.
Following are the attributes of a host name:
- Host name can contain maximum 255 characters
- Multiple host names can be assigned to single host
- Host name need not match the NetBIOS computer name (Microsoft Windows)
- Comments can be included by including a hash character (#)
Hosts file is located in:
|Microsoft Windows XP / 7||%SystemRoot%\system32\drivers\etc\hosts|
|Mac OS X 10.2||/private/etc/hosts|
DNS (Domain Name System)
Using HOSTS file for resolving names on large networks is practically impossible as the HOSTS file on each computer needs to be updated with entries of all other computers in the network an impossible task for public networks such as the Internet.
Hence, to address name resolution on large networks and the Internet, a hierarchical distributed naming system called as the DNS is used. Instead of storing information on each computer, entries are centralized to provide the name resolution for all computers in the network. This method helps reduce administrative costs and efforts, since only one machine has to be maintained for name resolution.
The method is analogous to retrieving phone numbers from a centralized service such as the Yellow pages instead of our personal phone books.
DNS provides a worldwide, distributed keyword-based redirection service and serves as an essential component for the functionality of the Internet. Unlike HOSTS file, DNS can be quickly updated and updates are distributed to other DNS servers across the globe.
Domain name space
Domain name space consists of trees of domain names and has multiple levels. For example, for a domain mail.google.com, .com refers to the top-level domain, google refers to second-level domain and mail refers to third-level domain. A single DNS zone may consist of one or more domains and sub-domains, Domain names are not case sensitive.
Top-level domains or TLD (A) are domains at the highest level in hierarchical Domain Name System of the Internet. For all levels, it is the last portion of a domain name. Management of most top-level domains is delegated to responsible organizations by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA) and is in charge of maintaining the DNS root zone. Second-level (B) domains are leased from a hosting provider for a fee. Third-level (C) domains are managed by administrators of the second-level domain and require no fee at all.
Internationalized country level code TLD’s are also available and are specially encoded domain names. These when viewed in web browsers, the contents are displayed in the native language such as Arabic, Chinese, etc..
Note: For complete list of domain zones, visit http://www.iana.org/domains/root/db/
DNS zones contain resource records which hold information associated with a domain name such as services. For example, MX records are used for mail servers, CNAME records are used for pointing to an alias such as www or blog (www.google.com, blog.example.com), etc.
Address resolution mechanism
When you use an application such as web browser or mail client, the domain names (e.g. Wikipedia.org or mail.google.com) are translated to an IP addres enabling your computer to communicate. Domain name resolvers determine the appropriate domain name servers responsible for the domain name to be accessed, by a sequence of queries, starting with the right-most (top-level) domain label.
The DNS process is explained below
- User opens an URL, www.example.org. Client sends a query to ISP’s DNS for the IP address of www.example.org.
- ISP’s DNS searches its database or cache to find matching IP address. If not found, query is forwarded to the root server.
- Root server traces the IP address of the .org DNS Server and sends it to ISP’s DNS Server.
- ISP’s DNS Server contacts the .org DNS Server by its IP Address. The DNS Server responds to ISP’s DNS Server with the IP address of www.example.org.
- Cllent communicates with www.example.org using its IP address.
Authoritative DNS Servers
Authoritative DNS Servers refers to DNS servers that have complete information about a domain and can provide answers to client queries directly.
Non-Authoritative DNS Servers
Non Authoritative DNS Servers may have a copy of the answers or cache copies of DNS queries and provide the answer to DNS clients.
NSLOOKUP (Name Server Lookup)
NSLOOKUP is a command line utility used for querying DNS servers.
Caching Name Server
Caching name servers store results of DNS queries for a period of time as per TTL (Time-to-live) configuration of each domain name record. Caching name servers can improve the efficiency of DNS traffic across the Internet and even increase the performance of end-user applications which use DNS. Caching name server need to be configured on the DNS Server.
DNS Client Resolver Cache (Microsoft Windows XP/7)
DNS client resolver cache is a RAM-based table that contains entries of Hosts file and host names that Windows has tried to resolve through DNS. The DNS client resolver cache stores entries for both successful and unsuccessful DNS name resolutions. This in turn can improve performance as Windows can locate the destination IP address directly from Cache (RAM) instead of initiating another query to an internal or external DNS server.
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