Internet names are the names which we use to refer to hosts on the Internet, such as www.tech-faq.com and www.freebsd.org.



Internet addresses are the numbers which routers use to move traffic across the Internet, such as 216.17.138.115 and 216.136.204.117.

nslookup

One of the best ways to understand DNS is to look at the DNS testing tool, `nslookup`.

Let's start by doing a forward DNS lookup on www.freebsd.org:

bash-2.05a$ nslookup www.freebsd.org

Server: localhost.net

Address: 127.0.0.1

Name: www.freebsd.org

Address: 216.136.204.117

Next, let's do a reverse DNS lookup on 216.136.204.117:

bash-2.05a$ nslookup 216.136.204.117

Server: localhost.net

Address: 127.0.0.1

Name: www.freebsd.org

Address: 216.136.204.117

DNS Record Types

DNS records which map names to numbers are known as "A Records" or "Address Records".

DNS records which map numbers to names are known as "PTR Records" or "Pointer Records".

Two other types of useful records which are stored in DNS are MX (Mail eXchange) Records and NS (Name Server) Records.

NS records show where the DNS servers are for a domain. Let's find IBM's name servers using `nslookup`:

bash-2.05a$ nslookup

Default Server: localhost.net

Address: 127.0.0.1

> set type=ns

> ibm.com

Server: localhost.net

Address: 127.0.0.1

Non-authoritative answer:

ibm.com nameserver = ns.watson.ibm.com

ibm.com nameserver = internet-server.zurich.ibm.com

ibm.com nameserver = ns.almaden.ibm.com

ibm.com nameserver = ns.austin.ibm.com

Authoritative answers can be found from:

ns.watson.ibm.com internet address = 129.34.20.80

internet-server.zurich.ibm.com internet address = 195.176.20.204

ns.almaden.ibm.com internet address = 198.4.83.35

ns.austin.ibm.com internet address = 192.35.232.34

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MX Records tell us where the e-mail servers are for a domain. This tells our mail systems where to send mail to. Let's use `nslookup` to find the mail systems for IBM:

bash-2.05a$ nslookup

Default Server: localhost.net

Address: 127.0.0.1

> set type=mx

> ibm.com

Server: localhost.net

Address: 127.0.0.1

ibm.com preference = 10, mail exchanger = e35.co.us.ibm.com

ibm.com preference = 10, mail exchanger = e1.ny.us.ibm.com

ibm.com preference = 10, mail exchanger = e2.ny.us.ibm.com

ibm.com preference = 10, mail exchanger = e3.ny.us.ibm.com

ibm.com preference = 10, mail exchanger = e4.ny.us.ibm.com

ibm.com preference = 10, mail exchanger = e5.ny.us.ibm.com

ibm.com preference = 10, mail exchanger = e6.ny.us.ibm.com

ibm.com preference = 10, mail exchanger = e31.co.us.ibm.com

ibm.com preference = 10, mail exchanger = e32.co.us.ibm.com

ibm.com preference = 10, mail exchanger = e33.co.us.ibm.com

ibm.com preference = 10, mail exchanger = e34.co.us.ibm.com

ibm.com nameserver = internet-server.zurich.ibm.com

ibm.com nameserver = ns.almaden.ibm.com

ibm.com nameserver = ns.austin.ibm.com

ibm.com nameserver = ns.watson.ibm.com

e35.co.us.ibm.com internet address = 32.97.110.133

e1.ny.us.ibm.com internet address = 32.97.182.101

e2.ny.us.ibm.com internet address = 32.97.182.102

e3.ny.us.ibm.com internet address = 32.97.182.103

e4.ny.us.ibm.com internet address = 32.97.182.104

e5.ny.us.ibm.com internet address = 32.97.182.105

e6.ny.us.ibm.com internet address = 32.97.182.106

e31.co.us.ibm.com internet address = 32.97.110.129

e32.co.us.ibm.com internet address = 32.97.110.130

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