This is most likely the anonymous naming service port. You need to fix it a value by editing the NamingService configuration in the default/jboss.jcml file. Change it to this and set the 'xxx' to the port you want to use:
does this affect the performance of the system? if you do not specify the port, the RMI will use more than one port. Is it because using one port is degrading the performance, or is it for another reason.