Resolv.getname

Hi,

I have been using Resolv.getname to verify that a callback has come
from our secure payments processor.

If I do:
nslookup 155.136.68.24

I get:

Non-authoritative answer:
24.68.136.155.in-addr.arpa name = wcc2.worldpay.com.

However if I do:
Resolv::getname('155.136.68.24')

I get:
irb(main):005:0> Resolv.getname('155.136.68.24')
Resolv::ResolvError: no name for 155.136.68.24

Can anyone shed any light or suggest a better way of verifying the ip
address?

Cheers
George