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Bronze

ping -a parameter

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.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: ping -a parameter

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

2 REPLIES
Hall of Fame Super Blue

Re: ping -a parameter

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

Bronze

Re: ping -a parameter

Thanks Jon.

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