Server only binds to IPv6?

[Nick-Zhang1996]

Hi somehow my server seems to bind only to IPv6 address. After I start the server on port 9999:

root@nickzhang:~# lsof -i:9999 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME acweb 1272 root 6u IPv6 292468 0t0 TCP *:9999 (LISTEN)

netstat gives same result. I confirmed my ipv4 port 9999 is open with netcat.

Any ideas?

Thank you!