. . . garage that is about 100 ft away from my house in the backyard.
With your garage having separate electric, connecting a cable for internet between the house and garage could result in a nasty ground loop and potentially fried equipment.
For example, if lightning strikes the ground near your property, it sets up a potential difference between the ground at the garage and the ground at the house. That potential difference causes current to flow between the two points via the path of least resistance. Often, the copper wires of the LAN cable will be that point of least resistance and the current is high enough to fry equipment on either end.
One solution is to connect the two grounding points together with sufficient cable size to avoid the potential difference and subsequent current flow. That solution may be impractical given the distance and cost.
Another option is an optical isolator to break up the current path. I've forgotten as to whether it's recommended to put optical isolators on both ends or if just one is good enough.
Going completely wireless is another option.
If you live in an area without a lot of lightning and few, if any, issues with surges on the power line, you may get by with disregarding this info. If you live in Florida (the lightning capital of the US), it's not a matter of if it will happen, it's a matter of when.
Your power company may have some good advice to offer on the subject if you can make contact with one of the engineers.
DC