SIP works by using a simple address format similar to HTTP and email. In its simplest format a SIP address on a LAN would be the devices IP address. This is because the simple notation, sip: 192.168.16.167 as an example, is all that is required to identify the recipient of the call. Even over the internet, this simplified format would work, though a more user friendly way would be to use the domain name rather than the public IP address and allow DNS to resolve the name for you. For example, sip: [email protected], would connect both devices which have public and reachable IP addresses – firewall rules allowing.
However, in practice this isn’t feasible as the vast majority of devices using IPv4 use private IP addresses that are not reachable over the internet. The solution therefore is to use a service provider who will provide a routing service and act as a proxy client. The first stage is that the user registers with the service provider and creates a username password combination, which will be used to identify and authenticate the SIP device and session. The SIP address will then look like [email protected], and by using this SIP address others will be able to reach you on the internet.
Service provider routing services are normally free, so registering and obtaining a generic SIP address is easy. If however you have your own domain name and a matching email address, that have a corresponding MX record that matches your networks public IP address, it is better to try to match them so that your SIP address would match your email address. This is easy to do as DNS will resolve the domain name and so long as you have a unique SIP address within your LAN you will be reachable from anywhere outside the network.