yes you need to forward port 81 to your server!
For normal http you should forward port 80
Also if you want to use https you should forward port 443
If you need to use FTP you must forward port 21
Of you use SSH you need port 22 as well
did i miss anything?
Yes, port 25 for SMTP,
port 110 for POP3,
port 995 for POP3s,
port 143 for IMAP,
port 993 for IMAPs.
Maybe port 53 for DNS.