I know this is an old post, but I had the same issue where I was able to get the Printer Port name, but not the IP. In my case I couldn't rely on the Port Name being IP_[IP Address] but found how to get hold of the actual IP from the port name.
Windows stores the information about ports in the registry under
HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors\Standard TCP/IP Port\Ports\[port name]
This key contains the values set up in the port configuration page, including IP address and port number.
A quick C# example to get the IP address
using Microsoft.Win32;
RegistryKey key = Registry.LocalMachine.OpenSubKey(@"System\CurrentControlSet\Control\Print\Monitors\Standard TCP/IP Port\Ports\" + printerPortName, RegistryKeyPermissionCheck.Default, System.Security.AccessControl.RegistryRights.QueryValues);
if (key != null)
{
String IP = (String)key.GetValue("IPAddress", String.Empty, RegistryValueOptions.DoNotExpandEnvironmentNames);
}