Basic Open Ports for W2000 Exch 2000

Windows 2000 Server SP2 and Exchange 2000 SP2
PC Client is Windows XP Pro joining the domain through My Computer properties, Name Tab.

I’m experimenting with security and have closed all but the following ports as a start.

With all the ports open, I can “join” the domain.

With only the following ports open, I can’t “join” the domain. Which one did I forget?

Enabled TCP/IP ports: 20 21 22 23 25 53 68 79 80 110 111 113 119 135 137 138 139 143 161? 443

There are obviously a fewmore port open than necessary so I can work through the problem for both Domain controllers with and without Exchange 2000. I’d like to repeat the project for Peer to Peer.

Thank you ….