Windows NT 4.0 allowed all numeric computer names. Windows 2000 does not.
Too many people where calling Microsoft support when PING 123 didn't find the computer named 123. When you would type PING 123, the response was:
Pinging 0.0.0.123 with 32 bytes of data: Request timed out. Request timed out. Request timed out. Request timed out. Ping statistics for 0.0.0.123: Packets: Sent = 4, Received = 0, Lost = 4 (100% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0msIf you upgrade, Windows 2000 will allow your 123, but new numeric names can't be created.