Hello!
I think I might know what it is. Heres a typical server.properties:
#Minecraft server properties
#Wed Mar 07 20:37:02 CET 2012
allow-nether=true
level-name=world
enable-query=false
allow-flight=false
server-port=25565
level-type=DEFAULT
enable-rcon=false
level-seed=
server-ip=
max-build-height=256
spawn-npcs=true
white-list=false
spawn-animals=true
online-mode=true
pvp=true
difficulty=1
server-name=Unknown Server
gamemode=0
max-players=20
spawn-monsters=true
view-distance=10
generate-structures=true
motd=A Minecraft Server
If you look at it, you can see that on IP, theres nothing. if you enter your ext. ip there, it will fail.
#Minecraft server properties
#Wed Mar 07 20:37:02 CET 2012
allow-nether=true
level-name=world
enable-query=false
allow-flight=false
server-port=25565
level-type=DEFAULT
enable-rcon=false
level-seed=
server-ip= <--- This should be empty
max-build-height=256
spawn-npcs=true
white-list=false
spawn-animals=true
online-mode=true
pvp=true
difficulty=1
server-name=Unknown Server
gamemode=0
max-players=20
spawn-monsters=true
view-distance=10
generate-structures=true
motd=A Minecraft Server