What is DNS, and its Working?

19 May, 2012

Introduction

A key component of the Internet and how it works revolves around the Domain Name System, otherwise known as DNS. The underlying technology behind the Internet, is that when a computer needs to talk to another computer on the Internet, they communicate via the computer's IP Address. The IP Address is a unique set of numbers associated with a particular machine, which will be discussed in a separate article. An example of an IP Address is 216.213.19.27, which is the IP Address that corresponds to www.pcprompt.net.

As the predecessor of the Internet, ARPANET, grew larger, connecting to remote machines by their IP Address grew cumbersome. It became more and more difficult for people to remember the IP Addresses associated with the machines they were trying to reach that a system was created to translate easier to remember symbolic names to their equivalent numerical IP Address. Thus the Domain Name System was born.

Domain Name System

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. Every time a new domain, like pcprompt.net, 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 a 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.

Real Life Example

A lot of what has been discussed may be a bit confusing, so lets do a real life example. In the flowchart below labeled Figure 1, you will see a computer trying to connect to www.google.com and the steps it takes.

We will discuss these steps below:

1. A User opens a web browser and tries to connect to www.google.com. The operating system not knowing the IP Address for www.google.com, asks the ISP's DNS Server for this information.
2. The ISP's DNS Server does not know this information, so it connects to a Root Server to find out what name server, running somewhere in the world, know the information about google.com.
3. The Root Server tells the ISP's DNS Server to contact a particular name server that knows the information about google.com.
4. The ISP's DNS Server connects to Google's DNS server and asks for the IP Address for www.google.com.
5. Google's DNS Server responds to the ISP's DNS server with the appropriate IP Address.
6. The ISP's DNS Server tells the User's operating system the IP Address for google.com.
7. The operating system tells the Web Browser the IP Address for www.google.com.
8. The web browser connects and starts communication with www.google.com.

Conclusion

As you can see, the Domain Name System is essential in the use of the Internet. Without it, you would not be able to type in names, but would have to remember the numerical IP Addresses in order to get anywhere on the Internet.

Reference: Internet

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How DHCP Works

17 May, 2012

Dynamic Host Configuration Protocol (DHCP) is a client/server protocol that automatically provides an Internet Protocol (IP) host with its IP address and other related configuration information such as the subnet mask and default gateway. RFCs 2131 and 2132 define DHCP as an Internet Engineering Task Force (IETF) standard based on Bootstrap Protocol (BOOTP), a protocol with which DHCP shares many implementation details. DHCP allows hosts to obtain necessary TCP/IP configuration information from a DHCP server.

Benefits of DHCP

• Reliable IP Address configuration
• Reduced Network Administration

How DHCP Works

1.DHCP DISCOVER: The Dynamic Host Configuration Protocol (DHCP) client broadcasts a DHCP discover message on the network containing its MAC address and NetBIOS destined for UDP port 68 (used by BOOTP and Dynamic Host Configuration Protocol (DHCP) servers). This first datagram is known as a DHCPDISCOVER message, which is a request to any DHCP server that receives the datagram for configuration information.

2.DHCP OFFER: Each DHCP server on the network that receives the request responds with a DHCP offer message as broadcast (UDP port 67) to the computer that issued the DHCPDISCOVER. An offered the IP address and subnet mask is also included in the message. If the DHCP client device received multiple DHCPOFFER, the DHCP client accepts the first DHCP offer that arrives.

3.DHCP REQUEST: The Dynamic Host Configuration Protocol (DHCP) client accepts an offer and broadcasts a DHCPREQUEST datagram. The DHCPREQUEST datagram contains the IP address of the server that issued the offer and the physical address of the DHCP client. DHCPREQUEST message requests the selected DHCP server to assign the DHCP client an IP address and other TCP/IP configuration settings. DHCPREQUEST message also notifies all other DHCP servers that their offers were not accepted by the DHCP client.

4.DHCP ACK: When the DHCP server from which the offer was selected receives the DHCPREQUEST datagram, it constructs a DHCPACK datagram. This datagram is known as a DHCPACK (DHCP ACKNOWLEDGEMENT). The DHCPACK includes an IP address and subnet mask for the DHCP client. It may include other TCP/IP configuration information like IP addresses for the default gateway, IP addresses for DNS servers, IP addresses for WINS servers etc.

DHCP Terms and Definitions

Term	Definition
DHCP server	A computer running the DHCP Server service that holds information about available IP addresses and related configuration information as defined by the DHCP administrator and responds to requests from DHCP clients.
DHCP client	A computer that gets its IP configuration information by using DHCP.
Scope	A range of IP addresses that are available to be leased to DHCP clients by the DHCP Server service.
Lease	The length of time for which a DHCP client can use a DHCP-assigned IP address configuration.
Reservation	A specific IP address within a scope permanently set aside for leased use by a specific DHCP client. Client reservations are made in the DHCP database using the DHCP snap-in and are based on a unique client device identifier for each reserved entry.
Exclusionrange	One or more IP addresses within a DHCP scope that are not allocated by the DHCP Server service. Exclusions ensure that the specified IP addresses will not be offered to clients by the DHCP server as part of the general address pool.
APIPA	A TCP/IP feature in Windows XP and Windows Server 2003 that automatically configures a unique IP address from the range 169.254.0.1 through 169.254.255.254 with a subnet mask of 255.255.0.0 when the TCP/IP protocol is configured for automatic addressing, the Automatic private IP address alternate configuration setting is selected, and a DHCP server is not available. The APIPA range of IP addresses is reserved by the Internet Assigned Numbers Authority (IANA) for use on a single subnet, and IP addresses within this range are not used on the Internet.
Superscope	A configuration that allows a DHCP server to provide leases from more than one scope to clients on a single physical network segment.
Multicast IP	Multicast IP addresses allow multiple clients to receive data that is sent to a single IP address, enabling point-to-multipoint communication. This type of transmission is often used for streaming media transmissions, such as video conferencing.
Multicast Scope	A range of multicast IP addresses that can be assigned to DHCP clients. A multicast scope allows dynamic allocation of multicast IP addresses for use on the network by using the MADCAP protocol, as defined in RFC 2730.
BOOTP	An older protocol with similar functionality; DHCP is based on BOOTP. BOOTP is an established protocol standard used for configuring IP hosts. BOOTP was originally designed to enable boot configuration for diskless workstations. Most DHCP servers, including those running Windows Server 2003, can be configured to respond to both BOOTP requests and DHCP requests.

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