Moody, who became the first Englishman to be sent off at Twickenham, was handed the suspension by a Six Nations disciplinary committee at a hearing convened in London on Monday evening.
The 27-year-old Leicester flanker was sent off in the 77th minute after landing several punches during the fight which marred England's record 40-3 triumph over Samoa.
The initial flashpoint was Alesana Tuilagi's dangerous tackle on Mark Cueto, which saw the England winger taken-out in mid-air and land head first onto the pitch.
Cueto launched a verbal volley at Tuilagi and, when the Samoan responded with a punch, Moody, who had originally intervened as peace-maker, waded in and the melee escalated.
Tuilagi was shown a red card for the tackle and Moody received his marching orders moments later.
Tuilagi, Moody's Leicester team-mate, received a six-week suspension for striking an opponent and for his dangerous challenge on Cueto, while Samoan fly-half Tanner Vili received a two-week ban, also for a dangerous tackle on the Sale winger.
Moody now begins his second lay-off this season after already serving a six-week suspension for punching Leeds number eight Jordan Crane during a second-team game in September.
The maximum suspension for striking an opponent is one year and while a recent crackdown on violence by the game's authorities has resulted in tougher penalties, Moody's initial role as peacemaker is sure to have worked in his favour.
The Tigers forward will be available for England's opening Six Nations match against Wales on February 4 but his chances of making the line-up after missing out on so much game time - the ban ends the previous Sunday - are slim.
Leicester will be without Moody for the remainder of their European Cup pool matches, their Powergen Cup clash against Northampton on Saturday and four Guinness Premiership games.
Tuilagi will return in time for the Premiership clash with London Irish at the Madejski Stadium on January 8. All three players admitted to the offences and have the right of appeal.