Manual

69
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Switchboard is useful because it’s not always simple to connect two devices over the Internet which are essenally
“peers”. There are two major reasons for this.
First of all, to iniate a stream to a device over the Internet requires that you know its IP address. This is the number that
gets applied to the desnaon eld of the IP packet, so Internet routers can determine how best to send it along its way.
Every device that connects directly to the public Internet must have one.
However, when web browsing or sending email, this informaon is usually hidden from the user. In the tradional client/
server scenario, such as web browsing, a Uniform Resource Locator (URL) is used to represent the IP address of the web
page (which is decoded by a DNS server). Once a computer requests a web page from a web server, the web server
can automacally derive the reply address from the request and respond to it. So the tradional four segment decimal
address (e.g. 70.22.155.130) is completely obscured to the user.
Even if you know your IP address, it’s quite possible that address will change over me. This is because the vast majority
of internet users establish their addresses via DHCP, a protocol whereby a server (maintained by the ISP) will deliver one
of their available addresses to the client on inial connecon. That address is “leased” from the server for a parcular
me period. Aer the “lease” expires, the server is free to change it.
The commonly used Network Address Translaon (NAT) router adds to the confusion, making codecs even harder to nd.
Most LAN-based internet connecons (as opposed to computers connected directly to ISPs) actually negoate with a
local router containing its own DHCP server. This router assigns the LAN computer or device a “private” IP address.
We’ll cover more about the challenges of connecng codecs behind NAT routers shortly. For now, remember that one
of the problems NAT servers add is that the private IP address delivered to the codec (and the only address of which the
codec is aware) has no bearing on the public address seen from the Internet.
In extreme scenarios, several layers of address locality can be stacked, assuring that the IP address assigned to your
device is several degrees removed from the public IP address used for connecons. Also, each address in the stack is
temporary and able to change at any me.