Firewalls sit between a router and application servers to provide access control. Firewalls were originally used to protect a trusted network (yours) from the untrusted network (the Internet). These days, it is becoming more common to protect application servers on their own (trusted, isolated) network from the untrusted networks (your network and the Internet).
Router configurations add to the collective firewall capability by screening the data presented to the firewall. Router configurations can potentially block undesired services (such as NFS, NIS, and so forth) and use packet-level filtering to block traffic from untrusted hosts or networks.
In addition, when installing a Sun server in an environment that is exposed to the Internet, or any untrusted network, reduce the Solaris software installation to the minimum number of packages necessary to support the applications to be hosted. Achieving minimization in services, libraries, and applications helps increase security by reducing the number of subsystems that must be maintained. The Solaris Security Toolkit provides a flexible and extensible mechanism to minimize, harden, and secure Solaris systems.
Your Site Security Policy should provide direction on such issues.