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# Multicast Port Ranges

To cooperate with firewalls, you can explicitly specify ranges of ports for multicast transports to use instead of ephemeral ports.

## Background

Ordinarily, multicast transports use ephemeral UDP ports for data transmission and protocols. However, the operating system could assign ephemeral ports that are not open through a firewall, which interferes with transport operation.

To cooperate with firewalls, you can explicitly specify two ranges of UDP ports that are open through the firewall. In this configuration the transport binds ports from these ranges instead of requesting ephemeral ports from the operating system. Ensure that the ports you specify are open in both directions through the firewall.

## Port Ranges Correspond to Transport Abilities

If a transport definition configures one or more Listen Groups, the runtime transport binds one of the available ports in the transport definition's UDP Receiver Port Range. Conversely, if the transport definition does not configure a listen group, then the runtime transport does not bind a receiver port.

If a transport definition configures a Send Group, the runtime transport binds one of the available ports in the transport definition's UDP Sender Port Range. Conversely, if the transport definition does not configure a send group, then the runtime transport does not bind a sender port.

## Sizing

Ensure that the ranges you specify include enough ports.

Each application process binds at most one port from the transport definition's UDP Receiver Port Range, and at most one port from the transport definition's UDP Sender Port Range.

When many application processes run on a multi-core host computer, the port ranges must include enough distinct ports for all the processes on the host.

In contrast, when application processes run on separate host computers, they can bind the same UDP port numbers without collision.

## Disjoint Ranges

For best results, ensure that the port numbers in the UDP Receiver Port Range and the UDP Sender Port Range do not overlap. Overlapping ports could slow transport creation.

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