failed to determine external IP address. Other players might not be able to connect

[spacewars]

When I try to host a server this message pops up, whenever I try to join my friends server "couldnt establush network communication with server" pops up and i get disconnected, how do I solve this?

[Fel]

It means that your internet "box" (it's a router but many ISP call it box) doesn't let that port go through by default.

Unless you plan on playing with friends (in that case you can all use hamachi and connect "directly" through it instead of going with a public server) you will most likely need to port forward.

I don't use this myself since I can just go to the admin mode of my internet "box" and set it there directly but this site should have all you need to set a port forward for factorio:
https://portforward.com/factorio/

[Killcreek2]

Devs have added "connect via Steam" recently (in v0.17?), to help the few people who still have trouble connecting peer-to-peer normally (due to quirky routers etc) ~ by routing the connection through Steams servers (peer-server-peer).

Bit more info here on the blog: https://www.factorio.com/blog/post/fff-265

You could opt-in to the 0.17 beta (see pinned topic), and give it a try. Probably easier than trying to manually configure your router &/or port forward. GL!

[spacewars]

Thank you boys! The 0.17 thing solved it!