You should have opened and forwarded (in case of a router) three ports:
TCP port (standard: 4662)
UDP port 1 (standard: 4665, always TCP+3)
UDP port 2 (standard: 4672, mustn't be TCP+3)
(See
the wiki page about this topic as well.)
There are 3 reasons for a low-id:
1. You have a firewall on your local machine or on your router.
2. You haven't forwarded the necessary ports to your local machine using a router.
3. Your ISP (internet service provider, the company providing your internet connection) doesn't like file sharing and blocks certain ports or filters packages.