Here I will show an example, how to block port specific communication in an entire computer networks. You have to add an user defined protocol in ISA server to block those ports. you may ask now why so? Let me explain little bit.
A port is an application-specific or process-specific piece of software that serves as a communication endpoint used by transmission layer protocols of the internet protocol suite, such as TCP or UDP. The port numbers are divided into three ranges: the Well Known Ports, the Registered Ports, and the Dynamic and/or Private Ports. The Well Known Ports are those from 0 through 1023. The Registered Ports are those from 1024 through 49151. The Dynamic and/or Private Ports are those from 49152 through 65535.
Sometimes these port are used for evil purposes such as spreading viruses in local area network. One example would be conficker virus. It communicate via netbios port 135-139. Here is the “how to” screen shots to block these ports.
Open ISA Management Console>Task pan>Tool Box>Protocols
Select user-defined>New>Protocol> Type W32.conficker, click next
ISA Management Console>Task pan>Tasks>Create New Policy
Add User-defined policy i.e. W32.Conficker .
Remove All Users and click next>ok