This question has been created to help adjudicate the question of whether a Familiar may communicate with their master inside of a Bag of Holding.
Suppose that 5' away from a Wizard is a portal to the Feywild. On the other side of the portal, their familiar is 5' away from the corresponding portal. The distance measured between them, using the portal, is only 10'. Does this satisfy the requirements of Find Familiar in its stipulation that its features may be used while the familiar is within 100' of the caster, or does the fact that they're on separate planes of reality mean that their distance is greater than the "walking distance" through the portal?
While your familiar is within 100 feet of you, you can communicate with it telepathically. Additionally, as an action, you can see through your familiar's eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has. During this time, you are deaf and blind with regard to your own senses.
Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.
—Find Familiar, Player's Handbook, pg. 240