But there's not really a mapping between them. Combined with GP's suggestion that it isn't really necessary to understand IP addresses (GPS coordinates) it works well though - you have some address, you follow signposts starting from the bottom to the broader area, culminating in road name signs and a number on the door. (i.e. 'the town is this way' road sign is like 'authority for ycombinator.com is blah')
The important thing to understand is that the domain name system provides an authoritative source of information linked to the domain (DNS entries). One type of such information is IP addresses, but that is not really relevant in this context. I haven’t looked at the AT protocol, but in principle a domain doesn’t even need to have an IP address in order to provide information about handles and fediverse identities (only the DNS server needs an IP).
