DOCBOOK: How to markup a port number?

In my continuing quest to achieve the perfect markup for this documentation I'm writing, I find myself asking if there is markup designed for things like port numbers, and if marking up the port number is overkill (too much granularity).

1) Is something like this appropriate for marking up a port number:
<para> The PORT parameter in the Oracle connection information should point to port <systemitem class="resource">1521</systemitem></para>.

2) Is this overkill, or could there be a good reason for this, a "best practices for technical documentation" reason?

Thanks,

Mike