I presume that I am OK with WS Pilots' Ports....?
FS 2004.
Dial-up Modem, plugged straight into the 'phone line.
Windows XP Pro, (original, no Service Packs).
Windows Firewall box un-ticked.
Routers....none...!
LAN...none...
No probs so far with 23456...

...!
..so are there likely to be any probs using 23457 and 23458..?
Cheers...!
Paul.
P.S. Interestingly, after six years of attempting to use Multiplayer without success, the WS Pilots site is the only one that has actually worked faultlessly using Port 23456...

...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.