i knew this from a long time ago but had forgotten…so writing it down this time.

the person hosting a l4d game needs to go to the console (~ key) and enter the following:
sv_lan 1
sv_allow_lobby_connect_only 0
net_start
map <map name>

clients can connect with:
connect <host IP address>

game modes can be changed with:
mp_gamemode (survival, coop, versus)

difficulty changed with:
z_difficulty (normal, etc.)