My server is Dell R210 and it is colocated. I need to find out whether the iDRAC BMC on this particular box has dedicated ethernet connector or is it shared.
Also - do I always need separate IP address to be able to remotely power off the box using IPMI ? (asking because additional IP costs money, need to be sure there is no other option).
Dell servers come with a service tag (basically a serial number consisting of 7 figures and letters). If you enter this at Dell website you get a comprehensive list of installed hardware. This includes the DRAC. From there you can read the manual and see if it comes with the dedicated interface.
This assumes that you can not simply look at the server. Mine (A dell R300) came with 3 ethernet ports, one marked DRAC.
As for a separate IP: The DRAC is basically a mini computer on a plugin card. So yes, you would need a different IP for it. I think most people even connect it to a different (management) network. Either physically separated, or via VLANs. The last thing you want is to expose this interface to the public. Esp. if you do not change the default password (which is 'calvin')
iDRAC BMC is what previously was called BMC, that means it uses by default NIC1 and to use the BMC you need to configure an IP, if I'm not wrong I think by default is 192.168.0.120.
So, to Answer your questions, your iDRAC BMC uses a shared NIC and you need a separate IP to be able to manage the server through IPMI.
It's kind of a tough spot to be in - asking us for details of the hardware you've bought. I can't guess what options you may have picked. Page 16 of the hardware manual says that the ethernet port is for an idrac6 enterprise. Since it says both the idrac6 express and idrac6 enterprise are optional, I can't say which one you have, and if you have the express, the manual leads me to believe that it might use an existing on-board ethernet port. (MichelZ's comment/link confirms my thoughts here, and so does the idrac6 manual - only Enterprise gets a dedicated NIC.) The manual for idrac6 says that all versions can do remote power, so that answers your question on that score.
Perhaps you might want to drop a line to Dell to help you with this - their online chat tech support guys are pretty good, IME. Assuming you have a support contract on this server, of course.
Drac express uses shared but enterprise uses a dedicated port. Both will need a unique IP even though express will share a NIC. Since you are in a colo you might need their helpdesk to cable up the drac enterprise port if that is what you have.
Remotely booting an R210 with an Enterprise DRAC, the DRAC BIOS setup section doesn't actually state that it's the Enterprise version (unlike the web interface). However, the Enterprise model offers a choice of dedicated or shared NIC. If you have a friendly data centre techie to call on, they could cycle the server, hit when the DRAC section shows up, and let you have the settings. I'd guess that if the DRAC BIOS has a working Virtual Media Configuration section, it's an Enterprise model. The Express doesn't have remote media mounting capability, and only works via a shared NIC.