The verb han preceded by the word antar takes the affix ap and ghan is the substitute of the word han when the word so formed means 'a country'.,
As अन्तर् + हन् + अप् = अन्तर् = घन् + अ = अन्तर्घनः 'name of a country of Bahika'; (a porch or court, an open space before the house between the entrance door and the house). Some write the word with a ण as अन्तर्घणः, that is also valid.
Why do we say 'when meaning a country'? Observe अन्तर्घातो 'striking in the middle'.,