How to play Doom multiplayer without source port??
I want play DOSBOX Doom multiplayer
I bought Doom on GOG.com and my friend bought Doom on GOG.com too.
We want to multiplayer.. but I don't know how to multiplayer the game..

Is Doom's multiplayer setup program only supports LAN?
If I opened the Multiplayer game. How to my friend join my game as a client????
I don't know. Help me please........

Yes, IPX networking is LAN-based only. There should be a modem/null-modem option as well but it won't help you much in this aspect.

You should check DOSBox built-in IPX over UDP emulation. I never tried it personally, but it should work even over the Internet if your connection is reliable enough. It requires two things: static or session-static WAN IP address and a port forwarding on the server side. If this can't be done, you'd better seek for some VPN solution.
When connection between two DOSBoxes establishes, you run DOOM's network setup program normally on the both sides, just like during the usual LAN game, and get your multiplayer. It's possible to have more than two DOSBoxes connected simultaneously and have up to four player game in DOOM. One of the nodes must be the server where other will be connected to, though.

See also this post and this article at the DOOM Wikia.

And I'd still recommend to use something like Chocolate DOOM source port, since the original engine might have issues even if you manage to setup network game successfully. It by design assumes the fact that networking layer is reliable while Internet is not. Also overall game speed will be limited by the node with the slowest connection/highest ping. Because old engine implements peer-based multiplayer, not the client-server model.

because of a bug, ipx does not work at all and never has through the setup program. dm does not support deathmatch 2.0(stuff respawns). it might be possible to somehow set up an emulated modem connection through your ip addresses but, i do not know how. dosbox however can emulate serial cable through ip addresses and that works perfectly through setup. in dosbox conf, you can set serial to modem.