A Local Area Network Primer

HTML5 applications are keyed to a domain name or IP address. This means that when you install your application into your device's web browser, you will only be able to synchronize back to the original IP address or domain.

Most problems with Pendragon Forms VI configuration relate to network configuration. This document is a primer on IP networks to help you understand how to configure your system.

IP Networks

An IP network is a network that sends packets from one computer to another using an IP address. An IP address is usually written as a sequence of four numbers separated by periods, e.g., 12.34.56.78. Each number in the sequence is in the range 0 to 255.

Across the Internet there are "routers" that route packets of information according to the IP address on each packet.

There are fewer than 4 billion IP addresses that can be addressed this way, but many user computers that access the Internet don't get their own IP address. Instead they share an IP address.

IP Address Ports

While the IP address is generally sufficient to locate a machine on the network, the address by itself does not contain enough information to say which application or service is being requested. Most Internet servers support several different services, such as web pages, email, FTP, etc. To overcome this ambiguity, IP packets are also labeled with a port number. By convention, port number 80 is used for web pages, port 443 is used for secure web pages, port 25 is used for email, etc.

By default, Pendragon Forms VI uses port 8080 for its web server application.

IP Address Sharing

Home WiFi networks and many corporate networks have only one public IP address. However, the routers for these networks are configured to automatically provision local IP addresses to computers that connect inside the network. While these local addresses cannot be seen directly by other machines on the Internet, your router performs Network Address Translation (NAT), and makes your local IP address appear like a public address while you surf the Internet.

For example, while your home DSL or cable modem may have a public IP address of 12.34.56.78, all of the PC's or devices inside your home WiFi network will get addresses like 192.168.1.2, 192.168.1.3, or 10.168.1.2, 10.168.1.3, etc.

Typically, each time you connect to your local network or boot up your PC, your router will assign you the first available local IP address.

A public video on NAT:

Outbound Connections From The Local Network

When a PC inside your network reaches out to a web server, the routers between you and the greater Internet translate your local address into a request on the shared public address. This means that when you access a remote web server from a PC on the local network, the remote server sees your packets as if they originate from the public address of your router (e.g., 12.34.56.78). Routers are aware of the connection (because they do the translation from your local address to the public address), and so the router knows how to send packets back from the public server you're accessing to your workstation or iPad.

Inbound Connections To The Local Network

However, while your router knows how to maintain a connection that you initiate in your web browser on the local network, it does not automatically know how to deal with connections initiated by remote servers or devices. The NAT is a many-to-one mapping of multiple local IP addresses to the single, public IP address.

When a remote machine contacts your public IP address to initiate a connection, your router doesn't automatically know which local IP address to send the request to. To get around this problem, most routers have a feature called Port Forwarding. Port Forwarding allows you to designate which public ports go to which PC's on the local network. To make port forwarding work, the destination PC on the local network needs to have a fixed local IP address.

Domain Names

It would be difficult to use the web if we had to remember sequences of numbers as addresses. It's much easier to refer to a web site by a domain name, e.g., www.yahoo.com. To make this work, there are servers on the Internet called Domain Name Servers (or DNS servers) which map names onto numbered IP addresses.

A public video on DNS:

Many of the secirity features of web browsers rely on the domain name. For example, web browsers store little bits of information about each web site on your local PC. These "cookies" may include credentials needed to keep you logged into a remote server. To prevent these cookies from being available to other public servers, the web browser makes sure that only the site that created the cookie can get access to it.

The data that Pendragon Forms VI stores in your browser is protected in the same way. Your stored data can only be accessed or synchronized to the original site (domain name or IP address and port) where the pages originated.

Secure Web Connections

Usually, when you access a web site in your browser, you type http:// followed by the domain name or IP address. HTTP is the web page protocol (HyperText Transfer Protocol). By default, HTTP is served on port 80.

To make web transactions secure, a different protocol, HTTPS, is used. HTTPS relies on a certificate-based encryption method. This method encrypts your data so that it cannot be read by anyone but you and the remote server, and it also authenticates the remote server as legitimate. For example, it prevents someone from getting between you and your bank's web server, and pretending to be your bank as you enter your password.