nslookup
Kloth.net (lookup by type MX, NS, ...) Gives details from IP
Multiple DNS Lookup engine (lookup a range of IP's)
InfoBear good for domain to IP

Some Samples
www.att.net 204.127.129.1 204.127.160.1
www.attglobal.net 165.87.13.127 165.87.201.244
www.bellatlantic.net (verizon) 151.196.0.38 151.196.0.39
www.aol.com
www.earthlink.com  207.217.126.81 207.217.77.82 207.217.120.83
www.uu.net www.wcom.net 198.6.1.5 198.6.1.4 

Dynamic Domain Name System (DDNS)

 DDNS lets you assign a fixed host and domain  name to a dynamic Internet IP address. It is useful when you are hosting your own website, FTP server, or other  server behind the Router.

 DDNS service providers e.g.  DynDNS.org or TZO.com, will adjust your IP address as it changes. 
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