The computer, tablet or phone you sit in front of is usually called “the
client” and the machine out there somewhere that you want to exchange data
with is called “the server”. The main difference between the client and the
server is in the roles they play here. There’s nothing that prevents the roles
from being reversed in a subsequent operation.

When you want to initiate a transfer to one of the machines out there (a
server), you usually don’t know its IP addresses but instead you usually
know its name. The name of the machine you will talk to is embedded in the URL
that you work with when you use curl.

You might use a URL like ““, which means you will
connect to and communicate with the host named example.com.

Host name resolving

Converting the name to an IP address is often called ‘name resolving’. The name
is “resolved” to a set of addresses. This is usually done by a “DNS
server”, DNS being like a big lookup table that can convert names to
addresses—all the names on the Internet, really. Your computer normally
already knows the address of a computer that runs the DNS server as that is
part of setting up the network.

curl will therefore ask the DNS server: “Hello, please give me all the
addresses for example.com”, and the server responds with a list of them. Or in
the case you spell the name wrong, it can answer back that the name doesn’t
exist.

With a list of IP addresses for the host curl wants to contact, curl sends out
a “connect request”. The connection curl wants to establish is called TCP and
it works sort of like connecting an invisible string between two
computers. Once established, it can be used to send a stream of data in both
directions.

Connects to “port numbers”

When connecting with TCP to a remote server, a client selects which port
number to do that on. A port number is just a dedicated place for a
particular service, which allows that same server to listen to other services on
other port numbers at the same time.

Most common protocols have default port numbers that clients and servers
use. For example, when using the “http://example.com/index.html“ URL, that URL
specifies a scheme called “http” which tells the client that it should try TCP
port number 80 on the server by default. The URL can optionally provide
another, custom, port number but if nothing special is specified, it will use the
default port for the scheme used in the URL.

After the TCP connection has been established, many transfers will require
that both sides negotiate a better security level before continuing, and that
is often TLS; Transport Layer Security. If that is used, the client and server
will do a TLS handshake first and only continue further if that succeeds.

Transfer data