I'm not sure if this is the right place to ask, but I have to start somewhere.

Background
For the last couple of years I've been successfully running a 3rd Party sip based PBX behind a 2850. However, the vendor has upgraded the software but advised that before I install I will now need a FQDN. I want to upgrade.

My issue is simple, but I lack sufficient understanding of DNS to complete the task.

I have a domain name say 'xyz.com'. I have set XYZ.com DNS A record to my static ip address say 65.45.46.47

However,a FQDN seems to be in the format of say PBX.XYZ.com. My assumption is the PBX part is a bit like saying use port 1010, but I don't know if this is correct and if it is, how do I set it up and if it isn't what do I now need to do?

Any help greatly appreciated.
Iain

If your sip provider requires the FQDN for that particular pbx then you would need to add a second DNS record at your hosting/domain registrar.

If you were a small VoIP company (for example) then you would likely be running more than one PBX and the chances are you would run each PBX on a different connection (for redundancy, failover, load balancing etc). For the sake of keeping things tidy (from your customers point of view) you would create a FQDN DNS A Record pointing to each separate PBX.

EG:
pbx1.fqdn.com - DNS A Record pointing to your WAN IP
pbx2.fqqn.com - DNS A Record pointing to your next WAN IP
pbx3.fqdn.com - DNS A Record pointing to another WAN IP
pbx4.fqqn.com - DNS A Record pointing to another WAN IP

This way each domain name will direct only to that server.