In Client/Server systems, the Server is usually at a
well-known address. Clients are configured with this
address.
In Peer-based systems, finding peers is somewhat more difficult.
A central server can hold a list of peers.
At startup, peers ask the server for addresses of
other peers. More than one such server is recommended
to avoid having a single point of failure.
On the public internet, it is possible to register
with existing services, like IRC or voice chat
systems to share one's IP address with other peers.
On a small broadcast network, peers can send
broadcast messages to locate one another.