04-24-2010 11:22 AM - edited 03-06-2019 10:47 AM
Hi everbody.
I was doing ping with parameter -a, when i encountered something strange. here is what i did.
ping help.com
I got the ip address from above command as 216.239.116.121
Then i used the command :
ping -a 216.239.116.121
My understanding was ping will also give me host name because we are using parameter - a, it did give me host name as:
c13-ss-help-www-lb.cnet.com.
I performed ip look up using http://ip-lookup.net/?216.239.116.121
I found entry as:
216.239.116.121--------- c13-ss-help-www-lb.cnet.com
Here is my question, when i clicked this link i was able to get to www.help.com ,thereby proving the ip address 216.239.116.121 is indeed mapped to www help.com. But when i moved the cursor over cnet.com" right after the period between lb and cnet in the link, i was directed to www.cnet.com.
They are two complete host name mapped to 216.239.116.121. How is it possible?
Also when i pinged www.cnet.com i get the ip address as 216.239.116.40 . But as mentioned above www.cnet.com is mapped to 216.239.116.121
May be i am not reading the mapping " 216.239.116.121--- c13-ss-help-www-lb.cnet.com" properly
thanks and have a good weekend.
Solved! Go to Solution.
04-24-2010 11:48 AM
Sarah
These are the DNS lookups for the entries you are asking about -
121.116.239.216.in-addr.arpa. PTR 300 c13-ss-help-www-lb.cnet.com.
www.help.com. CNAME 300 c13-ss-help-www-lb.cnet.com.
40.116.239.216.in-addr.arpa. PTR 300 c13-sha-stack2-netscaler2.cnet.com.
Note the second entry for www.help.com is a CNAME and not a PTR like the other 2. A CNAME or Canonical Name record is simply a way to map one DNS name to another so if you enter "www.help.com" then a DNS lookup is done and the response is c13-ss-help-www-lb.cnet.com. This response will then be looked up again to get an IP address ie. the first entry above will provide this.
So it's simply a way of having multiple "names" mapped to a DNS names eg. www.help.com, ftp.help.com, smtp.help.com could if you wanted all be mapped to c13-ss-help-www-lb.cnet.com. And that is why you are seeing multiple names mapped to the same IP.
Jon
04-24-2010 11:48 AM
Sarah
These are the DNS lookups for the entries you are asking about -
121.116.239.216.in-addr.arpa. PTR 300 c13-ss-help-www-lb.cnet.com.
www.help.com. CNAME 300 c13-ss-help-www-lb.cnet.com.
40.116.239.216.in-addr.arpa. PTR 300 c13-sha-stack2-netscaler2.cnet.com.
Note the second entry for www.help.com is a CNAME and not a PTR like the other 2. A CNAME or Canonical Name record is simply a way to map one DNS name to another so if you enter "www.help.com" then a DNS lookup is done and the response is c13-ss-help-www-lb.cnet.com. This response will then be looked up again to get an IP address ie. the first entry above will provide this.
So it's simply a way of having multiple "names" mapped to a DNS names eg. www.help.com, ftp.help.com, smtp.help.com could if you wanted all be mapped to c13-ss-help-www-lb.cnet.com. And that is why you are seeing multiple names mapped to the same IP.
Jon
04-24-2010 01:40 PM
Thanks Jon.
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