Internet drops with manual IP

I've just upgraded from a macbook mid-2007 to a new MBP (13" / 2.8Ghz). Same physical location, peripherals, cables, home LAN hardware & settings (= AEBS ,Ethernet, manual IP).

Now ...

• set it all up last night including migrating data from old to new - so same settings for most things, including network prefs. All worked fine.
• worked fine at first this morning too, but then suddenly ...
• lost internet connection through ethernet
• found I could get internet back using wifi only; also that I can still share files with other computers on the LAN through Ethernet; and other computers in the house are doing fine with Ethernet & manual IP settings
• [more fiddling ... ] seems internet will always drop out with manual IP, but the same manual IP works if I select "using DHCP with manual address" as distinct from "manually".
• had a look at a page on Apple's knowledge base, which seems to suggest that "using DHCP with manual address" is not really applicable if using an AEBS and that if you did it you would not have an internet connection!!

If it works like this, I guess I'm happy. But why!!!??? Can anyone enlighten me?