DCOM (Distributed Component Object Model) is a framework used by Windows to allow COM components to work over the network.
Unlike your traditional TCP/IP and UDP/IP services where a single protocol has a fixed port DCOM dynamically assigns ports for the COM objects it remotes.
Any client wishing to communicate with objects owned by the server will always connect to the same TCP or UDP port. Clients discover the port associated with a particular object by connecting to and using the services provided by DCOM’s Service Control Manager (SCM).
The SCM always operates at a fixed network port on every computer; this is always port 135 for both TCP and UDP.
DCOM is a great technology for what it was designed for but unfortunately that design did not include the internet or firewalls.
If a firewall exists between the client and the DCOM endpoint you must open up a large range of ports that can be used for virtually any traffic between those two endpoints. By default, those ports port range from 1024 to5000.
The nature of this model means that one port is allocated per process, so the number of available ports equates to the number of simultaneous DCOM processes allowed through the firewall.
This design basically negates the value proposition of the firewall; as such deployment of services based on DCOM across firewall boundaries is not something that should ever be done.