Norwich gained revenge for their 7-1 defeat to Colchester on the opening day of the season.
The U’s defending was awful throughout and they were punished by a Norwich side gunning for the League One title.
They fell behind after 16 minutes when Chris Martin fired past Ben Williams after defender Danny Batth failed to clear.
Martin doubled his side’s advantage on the stroke of half-time, before Gary Doherty made it 3-0 with an emphatic finish on 49 minutes.
Magnus Okuonghae fouled Grant Holt in the box but Wes Hoolahan missed the spot-kick, although Oli Johnson heaped more misery on the hosts with an 81st minute strike.
Holt rounded off the victory late on as former City player Ian Henderson saw red for lunging at Darel Russell.