Outside Aggression

[Almond808]

I have a Great Dane that just turned 4 this summer he has always been very well behaved until about a month ago. I moved into a new apartment about 4 months ago and he just now started to bark at and attempt to go after other peoples dogs. I'm not sure what to do to stop this problem he's always been well socialized and never aggressive towards other dogs, just somewhat protective of his "mommy". Any suggestions would be greatly appreciated.