If you want to block some connections from specifically ip address and not use fire wall, you can add some parameters in the sqlnet.ora that can achieve what you want.
for example:
/u02/oracle/product/9.2.0/network/admin/sqlnet.ora
# Generated by Oracle configuration tools.
NAMES.DEFAULT_DOMAIN = localdomain
NAMES.DIRECTORY_PATH= (TNSNAMES, ONAMES, HOSTNAME)
#This turns on the hostname/IP checking for your listeners
tcp.validnode_checking=yes
#invited nodes of the ip address
tcp.invited_nodes=(192.168.11.89,192.168.11.3)
#exclude a list of clients
tcp.excluded_nodes=(192.168.11.100)
remember,you need restart listener.
Just a small tip -:)
for example:
/u02/oracle/product/9.2.0/network/admin/sqlnet.ora
# Generated by Oracle configuration tools.
NAMES.DEFAULT_DOMAIN = localdomain
NAMES.DIRECTORY_PATH= (TNSNAMES, ONAMES, HOSTNAME)
#This turns on the hostname/IP checking for your listeners
tcp.validnode_checking=yes
#invited nodes of the ip address
tcp.invited_nodes=(192.168.11.89,192.168.11.3)
#exclude a list of clients
tcp.excluded_nodes=(192.168.11.100)
remember,you need restart listener.
Just a small tip -:)