How to open TCP port in Ubuntu 10.10?

sudo iptables -A INPUT -p tcp --dport (port number) -j ACCEPT

Shooting in the darkness:

sudo ufw allow 80

Now there is a rule in your ubuntu firewall allowing external access to your 80 port. If you want more specific rule see man ufw. If you are on non-routable IP address you have to forward port in your router. If you provide more context to your question, then I can adjust this answer.

Your question needs a lot more detail:

• What do you mean by "open a port"? Do you want to do it in a program? Just run a program?
• What is your network configuration? LAN structure, Internet uplink, OSes involved, which router/switch...
• Do you have any firewalls / NAT in place? On your system, elsewhere in the network?

That said, if you have the "typical" home setup (one or more computers hooked up to a combined switch/router with Internet uplink via DSL or cable): Typically the switch/router/modem combo has a built-in firewall. You'll probably need to open a port there. Usually these devices have a web interface for configuration, look into your manuals.

I suggest Netcat: The very basic way to do that (via Netcat) is:

nc -l <port_number>

Example:

nc -l 12569