Can one grant or restrict access to a system via SQL*Net?

Yes, add the following parameters to your sqlnet.ora file:

tcp.validnode_checking = yes
tcp.invited_nodes = (139.185.5.111)
tcp.excluded_nodes = (133.17.15.21)

The first line enables this features. The invited nodes is a list of the hosts, either DNS name or IP address, that are allowed to connect. The excluded nodes is a list of hosts that are not allowed to connect to the database.

Note: The tcp.invited_nodes parameter takes precedence over the tcp.excluded_nodes parameter if both lists are present.

Note 2: For Oracle 8i and below these parameters needs to be set in a protocol.ora file on your server (located in $ORACLE_HOME/network/admin or $TNS_ADMIN).