How do I assign reverse DNS?

I have a customer who has an ip block of 123.123.123.0 -- 123.123.123.9 and he wants to point reverse authoirty of the block to 123.123.123.10.

How is that done?

You should contact arin.net to move reverse DNS records to your DNS servers (you should have full C address block) and then set-up reverse (PTR) records/zones on your DNS server and you are done ;)

Enjoy

Er, is there a nameserver on the .10?

I'd be very careful about doing that, as sometimes spammers will ask to have control of their rDNS in order to spoof the identity of a legitimate company. Using an rDNS of mail10.hotmail.com, for instance.

You need to talk to your hosting provider to have them set-up CNAMES for the reverse.