1. You can check out the open files of a process:

[wh5a@pri-036 wh5a]$ ps -p 10189

PID TTY TIME CMD

10189 pts/0 00:00:00 a.out

[wh5a@pri-036 wh5a]$ ls -l /proc/10189/fd

total 0

lrwx------ 1 wh5a wh5a 64 Nov 10 00:32 0 -> /dev/pts/0

lrwx------ 1 wh5a wh5a 64 Nov 10 00:32 1 -> /dev/pts/0

lrwx------ 1 wh5a wh5a 64 Nov 10 00:32 2 -> /dev/pts/0

lrwx------ 1 wh5a wh5a 64 Nov 10 00:32 3 -> socket:[8081190]

2. Now you may wonder what the number 8081190 means. It is the inode assigned by the OS for it:

# output all tcp conns with extended info

[wh5a@pri-036 wh5a]$ netstat -tae

Active Internet connections (servers and established)

Proto Recv-Q Send-Q Local Address Foreign Address State User Inode

tcp 0 0 *:36864 *:* LISTEN wh5a 8081190

tcp 0 0 *:32768 *:* LISTEN rpcuser 2118

..........

3. Say you want to know which process opened port 36864 (inode # also shown):

# the manpage of lsof has more info

[wh5a@pri-036 wh5a]$ sudo lsof -i tcp:36864

COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME

a.out 10189 wh5a 3u IPv4 8081190 TCP *:36864 (LISTEN)