Dynamic DNS is exactly what it says it is.

Dynamic - Characterized by continuous change, activity, or progress: a dynamic market

DNS - Domain Name System (A method for a client to ask where Yahoo.com exists, and its "address")

Introduction:
Dynamic DNS is for machines that have a changing "address", such as modem (dial up/DSL/cable) user who wants to run his or her own server. Modem providers give out different IP's usually because they assume that a.) General User is not going to be online all the time, and if they have 1000 users, and 250 dialup modems with ip's, that should be enough to get by, and b.) with modern providers it provides a bit of security for users.

Nitty Gritty:
Regular DNS systems are designed with the idea in mind that IP addresses dont change often. Therefor when DNS entries are created on the main server, they are allowed to define certain time-to-lives in which a second dns server can give out information that it already had known. This is called caching. Usually domain name entries have time-to-lives of over a day. The secret to Dynamic DNS is that the time-to-live is set VERY low, often in the area of 5 minutes so that the entries are rarely cached, and every DNS server in the world has to ask the DNS server that holds your dynamic record what IP address it is pointing at EVERY time. This lets you change your IP address, yet still have a dynamic host.

Types:
There are two types of Dynamic DNS, Domain DNS, and Subdomain DNS.
Domain Dns allows you to have names such as mymachine.com point to your machine. This means you could possibly setup and run anything.mymachine.com and get email at whatever@mymachine.com.
Sub-Domain DNS is yourhost.bigprovider.com. They might let you have e2joeuser.bigprovider.com, or joes-house.bigprovider.com. This easier for the company to setup, and easier to maintain.

Providers:
There are many providers of dynamic DNS services. For an extensive list, along with more information about Dynamic DNS please goto

http://www.technopagan.org/dynamic/.

Want to run your own?
Source and Client for a working DynDNS system at: http://gnudip.cheapnet.net/
This is a (partial, incomplete) list of dynamic DNS providers.

It's not plagarism if you wrote it... this is copied from my DDNS resources page at http://www.technopagan.org/dynamic/.

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