User guide

VOIP INFORMATION 183
To access the Internet, a computer or VoIP device must have an IP address.
So what do you do if your ISP (Internet Service Provider) has provided you
with only one IP address but you have more than one device that requires
Internet access? The solution is to give each of the devices on the LAN a
private IP address. A router makes them work by performing Network
Address Translation (NAT — See the next topic for details).
Addresses of the form 192.168.x.x, 172.16.x.x or 10.x.x.x are not recognized
on the Internet and can only be used for private networks. For example, if
you tell someone on the Internet to connect to you using a 192.168.x.x
address, it will not work. Instead, you must provide your real/public IP
address.
What is NAT and how does it affect VoIP?
Network Address Translation (NAT) allows multiple devices to share the same
external IP address to access resources on the Internet. NAT is typically used
to allow all the devices in a subscriber’s local area network to access the
Internet through a router with a single public IP address assigned by the
Internet Service Provider.
If a VoIP device is sitting behind a NAT, the private IP address assigned to it
is not usable for communications with the entities outside the private
network. The VoIP device must substitute the private IP address information
with the proper external IP address/port in the mapping chosen by the
underlying NAT to communicate with a particular public peer address/port.
TalkSwitch can automatically check for the current public IP address and
substitute the private IP address with the public address so that VoIP traffic
is properly routed through the NAT.
What’s the difference between a static IP address and a dynamic
IP address and what effect does this have on VoIP?
A static IP address is an address that is permanently assigned to a device.
Typically, a static IP address has to be assigned by the network administrator
or Internet Service Provider (ISP).
A dynamic IP address is an address that is temporarily assigned to devices by
a DHCP or PPP server that maintains and assigns a pool of IP addresses.
When deploying VoIP devices, it’s best to have a static public IP address. This
prevents destinations from becoming unreachable for periods of time when
the IP address changes. But since this can be an expensive option,
TalkSwitch supports the ability to handle dynamic public IP addresses for
the Internet connection. TalkSwitch automatically checks for the current
public IP address and substitutes the private IP address with the public
address, so that VoIP traffic is properly routed through the NAT.