Here’s how to find out the IP address of the DHCP Server your Mac is currently using to obtain it’s IP address.

ipconfig getoption en0 server_identifier

You can also do more with ipconfig, things like determine the router
address for an interface:

ipconfig getoption en0 router

this is sure easier than what I used to do:

netstat -nr | grep default | awk '{print $2}';

this method, however, returns the system-wide default router whereas the ipconfig method returns the router for an individual interface.

replace en0 for en1 if using AirPort…