I have installed Jenkins and opened port 8080 on Ubuntu 12.04 server but still can't access Jenkins remotely. Below is the procedures I took.

# Install Jenkins, enable UFW and open port 8080
sudo apt-get install jenkins
sudo ufw enable
sudo ufw allow 8080
sudo ufw reload

# Check the status
sudo ufw status
8080    ALLOW    Anywhere
8080    ALLOW    Anywhere (v6)

# Locally
curl -I localhost:8080
HTTP/1.1 200 OK
Server: Winstone Servlet Engine v0.9.10
...

# On an external machine
curl -I [ip]:8080
couldn't connect to host