How can I create a listen server?

i was trying to find out how you can create a listen server and what ports need to be open, i googled it and it was just about dedicated servers, so can somebody please tell me how i can make one and what ports need to be open?

and btw, no, im not going to use hamachi

EDIT:
i want to make a server for gmod preferbly

I think you need to forward the same ports, and when you are in your server, write this in console:
¨sv_lan 0¨ and ¨heartbeat¨ without the quotes

Hope it works!

tried portforwarding and those commands, still dosent work

I don’t know about your modem or if your using a router but i had to portfoward on both the router and the modem.

For some info, listen servers use more then ports 27015…They use 27015,27020,27039,1200,27005. those must be open for it to work. If it still doesn’t your router isn’t letting it through

thanks, ill reply if it works!

[editline]06:12PM[/editline]

im not sure if ive portfowarded the modem, ill check