What is PTR record?

A PTR record is essentially the opposite of an A record. A records resolve names to IP addresses. PTR records resolve IP addresses to names.

A record = give me a name I'll give you an IP

PTR record = give me an IP I'll give you a name

Try it out in windows:

ping "www.yahoo.com"

Here is what I got:

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F:\>ping www.yahoo.com

Pinging www-real.wa1.b.yahoo.com [69.147.76.15] with 32 bytes of data:

Reply from 69.147.76.15: bytes=32 time=14ms TTL=53

Reply from 69.147.76.15: bytes=32 time=11ms TTL=53

Reply from 69.147.76.15: bytes=32 time=65ms TTL=53

Reply from 69.147.76.15: bytes=32 time=12ms TTL=53

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now do a "ping -a IP-ADDRESS" which will spit you back the name attached to that IP

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F:\>ping -a 69.147.76.15

Pinging f1.www.vip.re1.yahoo.com [69.147.76.15] with 32 bytes of data:

Reply from 69.147.76.15: bytes=32 time=19ms TTL=53

Reply from 69.147.76.15: bytes=32 time=88ms TTL=53

Reply from 69.147.76.15: bytes=32 time=12ms TTL=53

Reply from 69.147.76.15: bytes=32 time=46ms TTL=53

Ping statistics for 69.147.76.15:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 12ms, Maximum = 88ms, Average = 41ms

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Your accessing PTR records to do this.

... and just so you beileve me try a "ping 69.147.76.15" and check the results

FYI: if your doing a ping -a to an intranet host you need to configure reverse lookup for it to work properly.