The short answer is you can't. ARIN only cares about /24s or bigger. If the network were a /24 or larger, then your customer would need to get an ASN (autonomous system number) and then you could register the network to them.
More info here: https://www.arin.net --Patrick Darden -----Original Message----- From: Deric Kwok [mailto:deric.kwok2...@gmail.com] Sent: Friday, June 17, 2011 12:34 PM To: nanog list Subject: whoi modify question Hi My boss wants me to resign part of ip /25 to customer For the whois record to this customer, how can I do it? Thank you