Nslookup

The conversion between domain names and IP addresses is done by a system of domain name servers. These servers are usually called by client programs, but they can also be queried manually, using the Unix or Windows XP command-line command nslookup.

The following examples show nslookup finding the IP address for sws.csudh.edu and then doing a reverse lookup on 155.135.1.163 which returns sws.csudh.edu. 

C:\>nslookup sws.csudh.edu
Server:  vnsc-bak-dsl.genuity.net
Address:  4.2.2.5

Non-authoritative answer:
Name:    sws.csudh.edu
Address:  155.135.1.163

C:\>nslookup 155.135.1.163
Server:  vnsc-bak-dsl.genuity.net
Address:  4.2.2.5

Non-authoritative answer:
Name:    sws.csudh.edu
Address:  155.135.1.163

The DNS server that found the answer was running on vnsc-bak-dsl.genuity.net which is on the backbone network my ISP uses. I ran the above demonstration at home.

I then ran the same demonstration on campus: 

C:\>nslookup sws.csudh.edu
Server:  ns.csudh.edu
Address:  10.17.0.40

Name:    sws.csudh.edu
Address:  10.26.0.157

C:\>nslookup 10.26.0.157
Server:  ns.csudh.edu
Address:  10.17.0.40

Name:    sws.csudh.edu
Address:  10.26.0.157

Since I was inside the campus firewall, I saw the internal, non-routable address of sws.csudh.edu. The result was returned by a DNS server running on ns.csudh.edu inside our firewall.

The external, routable IP address 155.135.1.163 corresponds to the internal, non-routable IP address 10.26.0.157 . Network address translation, NAT, is conversion between corresponding internal and external IP addresses.

Instead of the nslookup command, you can a Web service like DNSStuff.com to make domain name and other queries. (DNSStuff calls nslookup for you).

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