PORT=5432; netstat -an |grep ":${PORT} " | awk '{count[$5]++} END {for (host in count) print host, count[host]}'

To view the same over time:

PORT=5432; watch "netstat -an |grep \":${PORT} \" | awk '{count[\$5]++} END {for (host in count) print host, count[host]}'"