The Domain Name System is the system used on the Internet for the mapping of names, such as www.google.com, to IP Addresses such as 216.239.51.99. It is a hierarchical namespace structure designed to provide host to IP address name registration and resolution.

Every time a new domain is registered, that domain is entered into one of the 13 Root Servers spread throughout the world and overseen by an organization called ICANN. Because your domain is in one of these servers, it can be reached and understood by the rest of the users on the Internet.

Another key element of the Domain Name System are DNS Servers run by Companies and Internet Service Providers. Every time you connect to a site, you are asking your ISP‘s DNS Server to resolve, or convert, the hostname such as www.google.com to an ip address such as 216.239.51.99. If your ISP‘s name server is not working or can not be reached, then you will not be able to traverse the Internet using hostnames, but instead would have to use their IP Address equivalent.

Any time you connect to a site, your ISP DNS Server must find out what name server has the information about the domain for the site you are trying to reach. Your ISP‘s DNS Server will connect to a Root Server and ask it who the name server is that knows the information about the site you are trying to reach. The Root Server will tell your ISP‘s DNS Server what server they should next contact for information. Next your ISP‘s DNS Server will then contact the server that the Root Server told it to contact, where it will be given the IP Address associated with the site you are trying to reach.

The system outlined above provides a somewhat simplified scenario. The Domain Name System includes several other functions:

• Hostnames and IP addresses do not necessarily match on a one-to-one basis. Many hostnames may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites.

• Alternatively a single hostname may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly.

• Sender Policy Framework and DomainKeys instead of creating their own record types were designed to take advantage of another DNS record type, the TXT record.

DNS primarily uses UDP on port 53 [2] to serve requests. Almost all DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server. TCP comes into play only when the response data size exceeds 512 bytes, or for such tasks as zone transfer. Some operating systems such as HP-UX are known to have resolver implementations that use TCP for all queries, even when UDP would suffice.

Important categories of data stored in DNS include the following:

• An A record or address record maps a hostname to a 32-bit IPv4 address.

• An AAAA record or IPv6 address record maps a hostname to a 128-bit IPv6 address.

• A CNAME record or canonical name record is an alias of one name to another. The A record to which the alias points can be either local or remote - on a foreign name server. This is useful when running multiple services (like an FTP and a webserver) from a single IP address. Each service can then have its own entry in DNS (like ftp.example.com. and www.example.com.)
• An MX record or mail exchange record maps a domain name to a list of mail exchange servers for that domain.
• A PTR record or pointer record maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in the in-addr.arpa. domain that corresponds to an IP address implements reverse DNS lookup for that address. For example (at the time of writing), www.icann.net has the IP address 192.0.34.164, but a PTR record maps 164.34.0.192.in-addr.arpa to its canonical name, referrals.icann.org.
• An NS record or name server record maps a domain name to a list of DNS servers authoritative for that domain. Delegations depend on NS records.

• An SOA record or start of authority record specifies the DNS server providing authoritative information about an Internet domain, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.

• An SRV record is a generalized service location record.

• A TXT Record allows an administrator to insert arbitrary text into a DNS record. For example, this record is used to implement the Sender Policy Framework and DomainKeys specifications.
• NAPTR records (”Naming Authority Pointer“) are a newer type of DNS record that support regular expression based rewriting.

ICANN