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LAN vs. Internet
Most of the networking you’ll be dealing with will exist within your LAN (Local Area Network) and connections between
devices within the LAN follow ordinary rules to send packets between each other. But in the situation where you wish to
connect to a device outside the LAN (which is most common) special rules need to be followed.
LANs have IP addressing conventions that allow a range of addresses to be reused within the network, and prohibit those
addresses not to be used again on the public internet. This allows for many devices to site behind a router, which has a single
internet (publicly addressable) IP address, and each LAN device to have a private, reusable IP address. By convention the
address ranges start with the digits 192.168.x.x, 172.16.x.x, or 10.0.x.x. So, for example, if a machine tries to connect to
another at an address of 10.0.0.75, it is necessarily trying to send packets only within its LAN. The range of addressable LAN
addresses is called a subnet, and must be programmed into each machine using a subnet mask entry.
If a machine on a LAN wishes to send packets outside the subnet, it must communicate with a gateway (usually a router) at
a fixed IP address.
Network Address Translation
The concept of how a gateway router provides translation services to the Internet is extremely important in the field of VoIP,
if only because it causes so many headaches. Known as Network Address Translation (NAT), it’s easiest to use a diagram to
illustrate a typical gateway scenario describing a user on a LAN accessing a web page at comrex.com. For this illustration,
we’ll ignore the concepts of DNS and URLs (which aren’t particularly useful for VoIP) and live the fantasy that the user is
accessing the comrex.com page via its public IP address, which is (as of this writing) 64.130.2.52. In our scenario, the
user has a laptop on a LAN using the popular 192.168.0.x subnet addressing scheme, and specifically has the address of
192.168.0.42 assigned to it.
The user will input the web page address into his browser, and the computer will recognize the address as outside the subnet
it has been programmed to work on. So it will form a packet, whose payload consists of a request to view the web page, and
hand it to the gateway router, which is located at the local address programmed into the laptop (192.168.0.1).
Because the router is acting as a gateway, it actually has two IP addresses. The LAN address (192.168.0.1) is used by devices
on the LAN. The WAN address (74.94.151.151) is the address assigned by the Internet Service provider. This address is
public, in that it is addressable by every device on earth that is connected to the Internet.
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