Hi Takezo,
Like Xplodingbubbles shared below, a properly configured NAT is important for a few reasons. NAT is how well your console can talk to other consoles, which explains the trouble with joining fireteams. I hope you have the chance to read that help article.
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Edited by MindfulPizza: 12/18/2014 4:03:12 PMIt not the NAT that blocking people it the outdated modems and routers. and Some the Pc firewalls on the network, Even on consoles. What even worst is those people open there nat that way, now going be open to flaws .