Name resolution and Domain Name System in a Microsoft TCP/IP-based network

Name resolution and Domain Name System in a Microsoft TCP/IP-based network

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Name Resolution

In a TCP/IP network, communications are based on IP addresses. All devices in a network use IP addresses to communicate with each other. Computers or network devices (such as routers) process IP addresses easily and work accordingly, whereas human beings (or users) find it difficult to remember these 32-bit IP addresses. For example, it is difficult for a human being to remember 66.223.125.240 than www.uCertify.com for a communication address. This is why friendly names are associated with computers, sites, file system shares, e-mail addresses etc., so as to help users remember them. Whenever a user uses these friendly names to send information, the computer converts the names of the destination computer into IP addresses. This conversion is called name resolution.

Designing a Name Resolution Strategy

In a Microsoft Windows-based network, usually NetBIOS names and host names are associated with computers and other resources. These names are used for communication amongst themselves. Hence, NetBIOS and host names resolutions are required to be configured on the network.

Note: The Active Directory directory services of Windows Server 2003 and Windows 2000 Server use DNS names for communication.

WINS servers and DNS servers are used for NetBIOS and host names resolutions respectively. Although Windows releases after Windows 2000 are based on DNS name resolution, they include the WINS server and WINS client to interact with computers on the network running the older operating systems.

Host Name

A host name is a name of a computer or other network resources used by users for communication. At the initial stages of networking, host tables are used for host names resolution. A host table is a text file placed in each computer, which contains a list of host names and their equivalent IP addresses in the following format:

192.168.0.201	ucfileserver	# File Server
10.12.34.81	ucclient05	# client host computer
10.12.34.85	ucclient09	# client host computer
127.0.0.1	localhost

The first column of the table contains IP addresses, the second column contains their corresponding host names, and the third column contains a hash symbol (#) and administrator's comment. A computer ignores everything that is written after the hash symbol. Whenever a host name request is encountered, the computer refers to the host table for host name to IP address resolution. In Windows 2003, the host table is located in the %Systemroot%System32DriversEtc folder in the file named Hosts.

As the size of the host table grows with time, maintaining a host table in each computer becomes impractical due to the manageability factor. Hence, there has been a need to find a centralized service with distributed data for name resolution. This need has been fulfilled by the evolution of DNS.

Domain Name System (DNS)

Domain Name System (DNS) is a hierarchical naming system used for locating domain names on private TCP/IP networks and the Internet. It provides a centralized service for mapping DNS domain names to IP addresses and vice versa.

DNS consists of a hierarchical namespace, a collection of name servers, and DNS clients. DNS clients are also called resolvers. Each server in a collection contains information for a small part of the namespace, which is known as the authoritative source for that part of the namespace. Whenever resolvers send queries to the DNS server, the server tries to retrieve information from its own database to resolve the IP addresses associated with the request names. If the entries are not found in its own database, it passes on the requests to other DNS servers for name resolution.



DNS servers keep the information in the DNS database in the form of resource records.

Resource Record (RR)

A collection of information about domains is known as a resource record (RR). A DNS server contains various resource records about a domain in a DNS database. These records are used to process DNS client queries. Some of the important resource records are as follows:

• A: An A (host) resource record is a DNS database resource record (RR), which contains the owner name or host DNS name, and the host IP address. Multiple host records can be created in a DNS database with the same host name that maps to the IP addresses of different Web servers.


• MX: An MX resource record is also known as a mail exchanger record in the zone file of the Domain Name Server (DNS). An MX record associates the domain name to a domain name classified in an address record (A record). The syntax defining an MX record depends on the TCP/IP software run by the host.


• CNAME: Canonical Name (CNAME) is a resource record that creates an alias for the specified Fully Qualified Domain Name (FQDN). It hides the implementation details of a network from the clients that are connected to the network.


• SRV: An SRV resource record is a DNS record that enables users to specify the location of servers for a specific service, protocol, and DNS domain. For example, if there are two servers in a domain, creating SRV records specifies which hosts serve as Web servers, and resolvers can then retrieve all the SRV resource records for the Web servers.


• PTR: A PTR resource record, also known as a pointer record, is a record in the Domain Name System (DNS) database that maps an Internet Protocol (IP) address to a host name in the domain. PTR records are used to perform reverse DNS lookups.


• HINFO: Host Information (HINFO) resource records are maintained to provide hardware and operating system information about each host in the network. For security reasons, HINFO records are not provided on public network servers. No application depends upon these records, and the sole purpose of maintaining HINFO records is to provide additional information to the applications. For example, application protocols such as FTP may use special procedures when communicating with computers of a known CPU and operating system type.

Domains, Domain Namespace, and Name Resolution

Domains are usually computers, which are grouped together for administrative purposes. These domains have a domain name. The domain name part of a DNS name is hierarchical and can consist of two or more words separated by a dot (.). The domain namespace has a root at the top, and beneath the root there are top-level domains. Under these domains, there are second-level domains. This chain can be stretched to any extent. At the end of the chain is a host computer. For example, in the fully qualified domain name, www.ucertify.com, www is a host name (or computer) that is placed in the ucertify domain. The ucertify is a sub-domain of a top-level domain named com.

Each level of domains has its own DNS server to resolve host names. The domains at each level maintain information about the domains at the next lower level in the hierarchy. When a request comes to a DNS server and the server is unable to resolve the query, it forwards the request to a DNS server of the next higher level in the hierarchy. This process is known as referral.

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