dataguard and firewall

i am asked to come up with a HA solution for a database running in DMZ layer. 
i have to create a standby database in another network layer which is behind a 
firewall.

now my question is can it be done? if it can be, what ports needs to be opened 
up. DB is 9.2.0.6, 10GB in size

any help is appreciated.

Thanks
Deen

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