A public Radix network (Universe) is segmented into a very large shard space (currently 18.4 quintillion shards). The start and end point for any Atom in the Radix Universe is an address, which is formed of a public key and a Universe checksum. The shard number of an address is deterministically calculated by taking a modulo of a public key over the total shard space to derive the shard index. This makes it trivial to for anyone to correctly calculate the shard a public key lives on.
For more information, see: https://papers.radixdlt.com/incentives/#shards